{"site":"https://digitalgemology.com","build":"20261101-105320","name":"Gemology glossary","url":"https://digitalgemology.com/learn/glossary","topics":["Grading report","Four Cs","Anatomy and proportions","Clarity characteristics","Color and fluorescence","Light performance","How we measure","Optics","Gem testing","Gemstone properties","Colored gemstones","Treatments and origin","Diamond and simulants","Cuts and shapes","Cutting and files"],"terms":[{"term":"10x magnification","slug":"10x-magnification","aka":["10x","10x loupe","loupe","10 power"],"category":"Four Cs","definition":"10x magnification is the standard view for diamond clarity grading: ten times life size, with a loupe or a microscope.","detail":"A clarity grade describes what a skilled grader finds at 10x, not what you see at arm's length. Many inclusions that a grader finds at 10x are not visible to the unaided eye. A 10x loupe is also the first tool to look for doubled facet edges in moissanite.","on_report":"GIA and IGI both set the clarity grade at 10x.","related":["clarity-grade","clarity-plot","facet-doubling","polish"],"url":"https://digitalgemology.com/learn/glossary/10x-magnification","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"3D Gemstone Simulator","slug":"3d-gemstone-simulator","aka":["simulator","gemstone simulator"],"category":"How we measure","definition":"The 3D Gemstone Simulator is a free Digital Gemology tool that renders a cut stone as a camera sees it. You choose the cut, gemstone, carat weight, color, clarity, cut grade and lighting.","detail":"It is for seeing, not scoring. The only figures it reports are the Light Return shares, the cut grade proportions and the size. Its report is a simulation of the geometry, not a grading report.","how_we_measure":"The graphics card ray-traces each pixel with full Fresnel reflection, three-color dispersion and body color. Each path takes up to 14 bounces, and a still image refines over up to 64 samples per pixel. In doubly refractive stones it traces both rays, so moissanite's facet edges double. It has ten lighting scenes.","related":["light-return","light-map","ray-tracing","fresnel-reflection","doubly-refractive","simulator-cut-grade"],"url":"https://digitalgemology.com/learn/glossary/3d-gemstone-simulator","sources":["https://digitalgemology.com/tools/3d_gemstone_simulator","https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"3D model","slug":"3d-model","aka":["gem 3D file","3D gemstone model"],"category":"Cutting and files","definition":"A 3D model of a gemstone is a digital solid of its facets for CAD, rendering, printing or games. An exact model has flat facets, sharp edges and a closed, watertight surface at real size.","detail":"A model made to look good is not always made to be cut. Rounded, smoothed or open models cannot be measured. Use a cutting file to cut a stone, and a 3D model to set, render or print it.","how_we_measure":"Our models are in millimeters, with the table facing +Y and the girdle plane through the origin. They come as STEP, 3DM, STL, OBJ, FBX and GLB. We build them from the same geometry as the cutting files.","related":["mesh","step-file","watertight","convex","cutting-file"],"url":"https://digitalgemology.com/learn/glossary/3d-model","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/3d-models"]},{"term":"3DM file","slug":"3dm-file","aka":["3DM","Rhino file"],"category":"Cutting and files","definition":"3DM is the native file format of Rhino, the CAD program behind much jewelry design. Jewelry programs that run inside Rhino, such as MatrixGold, open it directly.","detail":"KeyShot also opens 3DM files. Our 3DM files hold the stone as a closed mesh on its own layer.","how_we_measure":"Our 3D files include 3DM. A browser cannot read it, so our tools do not open your own 3DM files. Export STL, OBJ, GLB or STEP instead.","related":["step-file","3d-model","mesh"],"url":"https://digitalgemology.com/learn/glossary/3dm-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"Abrasion","slug":"abrasion","aka":["abrasions"],"category":"Clarity characteristics","definition":"An abrasion is a row of tiny nicks along the facet junctions of a cut diamond. It makes the edges look white or fuzzy.","detail":"It is a blemish: it lies on the surface. GIA's polish grade covers surface condition, so abrasions can count there as well as in clarity.","on_report":"A blemish. Minor blemishes are often not plotted; Comments can mention them.","related":["nick","chip","blemish","polish"],"url":"https://digitalgemology.com/learn/glossary/abrasion","sources":[]},{"term":"Absorption line","slug":"absorption-line","aka":["absorption band"],"category":"Optics","definition":"An absorption line is a narrow dark line in a spectrum, where a stone absorbs a narrow range of wavelengths. An absorption band is a broad dark stretch where it absorbs a wide range.","detail":"Ruby's chromium R lines are a close pair at 692.8 and 694.2 nm. Sapphire's strongest iron line is at 450 nm. Emerald shows chromium lines at 683.0 and 680.5 nm. A yellow (Cape) diamond shows its N3 line at 415.5 nm.","how_we_measure":"The Spectroscope names each line and band and classes it by contrast. A line under 0.04 is too faint to see, under 0.15 weak, under 0.4 distinct, and strong above that. Bands use the steps 0.1, 0.35 and 0.7. These steps are our own choice, not a published scale.","related":["absorption-spectrum","spectroscope","beer-lambert-law"],"url":"https://digitalgemology.com/learn/glossary/absorption-line","sources":["https://digitalgemology.com/about/how-we-measure/spectroscope","https://digitalgemology.com/tools/spectroscope"]},{"term":"Absorption spectrum","slug":"absorption-spectrum","aka":["spectrum"],"category":"Optics","definition":"An absorption spectrum shows which wavelengths of light a stone absorbs. In a spectroscope, the absorbed parts of the spectrum look dark.","detail":"The pattern often names the coloring element, not the stone: chromium in ruby and emerald, iron in sapphire. A laboratory-grown ruby shows the same chromium lines as a natural one.","how_we_measure":"The Spectroscope, Dichroscope and Polariscope compute from measured polarized spectra. Ruby uses GIA's published Cr³⁺ cross-sections (Dubinsky and others, 2020). Sapphire, emerald and amethyst use G.R. Rossman's measured files from Caltech.","related":["absorption-line","spectroscope","pleochroism","beer-lambert-law","cie-colorimetry"],"url":"https://digitalgemology.com/learn/glossary/absorption-spectrum","sources":["https://digitalgemology.com/about/how-we-measure/spectroscope","https://digitalgemology.com/about/how-we-measure/dichroscope"]},{"term":"AGS Ideal Report by GIA","slug":"ags-ideal-report","aka":["AGS Ideal","AGS Ideal 0","AGS Ideal supplement"],"category":"Grading report","definition":"The AGS Ideal Report by GIA is a digital supplement to a GIA report. It gives the American Gem Society's light performance grade, and only AGS Ideal 0 stones get one.","detail":"A round must first be a GIA \"Triple Excellent\": Excellent cut, polish and symmetry, in the D-to-Z range. Some facet designs of the cushion, oval, emerald, radiant, princess and fancy round also qualify. They need Excellent polish and symmetry. Marquises and pears are not on the list, and the report comes with or without an ASET map.","on_report":"A separate supplement, not a line on the main report. On GIA paperwork, \"Ideal\" appears only here, never as a cut grade.","related":["ideal-cut","triple-excellent","aset-map","cut-grade","light-performance"],"url":"https://digitalgemology.com/learn/glossary/ags-ideal-report","sources":["https://digitalgemology.com/learn/grading-reports/what-ideal-cut-means","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report"]},{"term":"Amethyst","slug":"amethyst","aka":["purple quartz"],"category":"Colored gemstones","definition":"Amethyst is the purple variety of quartz. Iron in the crystal, together with natural radiation, causes its color.","detail":"Amethyst is 7 on the Mohs scale and has good toughness. It often shows color zoning, which you see best table-down on a white surface. Long exposure to strong light can fade its color, and sudden temperature changes can crack it. Heat can turn it yellow, and most citrine on the market is made this way.","on_report":"Labs identify it as (natural) quartz, variety amethyst. Laboratory-grown amethyst is named as such.","related":["quartz","citrine","ametrine","color-zoning","stability","hydrothermal-growth"],"url":"https://digitalgemology.com/learn/glossary/amethyst","sources":["https://digitalgemology.com/gemstones/amethyst","https://digitalgemology.com/gemstones/amethyst/asscher","https://digitalgemology.com/tools/dichroscope","https://digitalgemology.com/tools/polariscope"]},{"term":"Ametrine","slug":"ametrine","aka":["amethyst-citrine"],"category":"Colored gemstones","definition":"Ametrine is quartz that is part amethyst and part citrine: purple and yellow zones in one crystal.","detail":"Most natural ametrine comes from Bolivia. Laboratory-grown ametrine also exists.","related":["amethyst","citrine","quartz","color-zoning"],"url":"https://digitalgemology.com/learn/glossary/ametrine","sources":[]},{"term":"Appraisal","slug":"appraisal","aka":["valuation","insurance appraisal"],"category":"Grading report","definition":"An appraisal is a statement of what a stone or a piece of jewelry is worth. A diamond grading report is not an appraisal.","detail":"A grading report says what the stone is and how it grades. It gives no price. People often call a grading report a \"certificate\" and expect a value on it. A lab such as GIA, which doesn't buy or sell diamonds, gives none.","related":["diamond-grading-report","four-cs"],"url":"https://digitalgemology.com/learn/glossary/appraisal","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report"]},{"term":"As mounting permits","slug":"as-mounting-permits","aka":["mounted grading","graded in setting"],"category":"Grading report","definition":"\"As mounting permits\" means that a lab graded a stone in its setting. The grader could judge it only as far as the setting allowed.","detail":"IGI grades mounted stones this way. Color and clarity then become a range, such as D–E, with an estimated cut grade. GIA's diamond grading reports are for loose stones only.","on_report":"On IGI reports for set stones.","related":["diamond-grading-report","color-grade","clarity-grade"],"url":"https://digitalgemology.com/learn/glossary/as-mounting-permits","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report"]},{"term":"ASET map","slug":"aset-map","aka":["ASET","Angular Spectrum Evaluation Tool image","ASET image"],"category":"Grading report","definition":"An ASET map is a four-color picture that shows from which directions a stone's face-up light comes. It can be part of the AGS Ideal Report by GIA.","detail":"Red marks the strongest light, from high above. Green marks dimmer light that still adds sparkle, and blue marks the dark contrast areas. Gray marks light lost through the bottom, and a stone that scores well is mostly red. Our simulator's light map uses the same idea, but it is our own render and shows leakage in black.","on_report":"Only on the AGS Ideal Report by GIA, in the version with the map. For an oval, ask for that version to check for a bow-tie.","related":["ags-ideal-report","light-return","leakage","contrast","bow-tie"],"url":"https://digitalgemology.com/learn/glossary/aset-map","sources":["https://digitalgemology.com/learn/grading-reports/what-ideal-cut-means","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report","https://digitalgemology.com/learn/bow-tie/how-to-check-a-diamond-for-a-bow-tie"]},{"term":"Asscher cut","slug":"asscher-cut","aka":["Asscher","square emerald cut"],"category":"Cuts and shapes","definition":"An Asscher cut is a square step cut with deeply cropped corners, which give it an octagonal outline.","detail":"Its stepped pavilion gives a concentric \"hall of mirrors\" look face up. Like other step cuts, it shows color and clarity plainly.","on_report":"No GIA cut grade; named in the shape and cutting style line.","related":["step-cut","emerald-cut","fancy-shape","shape-and-cutting-style"],"url":"https://digitalgemology.com/learn/glossary/asscher-cut","sources":["https://digitalgemology.com/cuts/asscher"]},{"term":"Assembled stone","slug":"assembled-stone","aka":["doublet","triplet","composite stone"],"category":"Treatments and origin","definition":"An assembled stone is made of two or more pieces joined together, usually with glue or by fusing. A doublet has two parts; a triplet has three, or two parts with a colored layer between them.","detail":"The pieces can be natural or man-made. Many assembled stones imitate a more valuable gem.","related":["simulant","manufactured-product","synthetic-gemstone","disclosure"],"url":"https://digitalgemology.com/learn/glossary/assembled-stone","sources":[]},{"term":"Asterisk after the color grade","slug":"asterisk-color-grade","aka":["color grade asterisk","F*","treated color asterisk"],"category":"Grading report","definition":"An asterisk (*) after a GIA color grade means that the lab detected a color treatment. Comments explains what it is.","detail":"For example, HPHT processing can remove or change a diamond's color. GIA then marks the grade, describes the treatment in Comments and inscribes TREATED COLOR on the girdle. An asterisk without an explanation in Comments is a sign that something on the report is wrong.","on_report":"Directly after the color letter, for example \"H*\".","related":["color-treatment","hpht","comments","color-grade"],"url":"https://digitalgemology.com/learn/glossary/asterisk-color-grade","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/grading-reports/how-to-read-a-diamond-grading-report"]},{"term":"Asterism","slug":"asterism","aka":["star effect","star sapphire","star ruby"],"category":"Gemstone properties","definition":"Asterism is a star of light that glides across the dome of a gem. Light reflects from fine needles, often rutile, that lie in two or three directions.","detail":"Star sapphires and star rubies usually show six rays, and occasionally twelve. Some other gems show four. The stone must be cut as a cabochon to show the star. The finest stars are sharp, straight and centered.","on_report":"Labs name the phenomenon, for example star sapphire.","related":["silk","needle","chatoyancy","sapphire","ruby"],"url":"https://digitalgemology.com/learn/glossary/asterism","sources":[]},{"term":"Average diameter","slug":"average-diameter","aka":["mean diameter"],"category":"Anatomy and proportions","definition":"The average diameter of a round is the mean of its minimum and maximum diameters, measured across the girdle.","detail":"It is the base for the percentages on a report. Table, crown height, girdle, pavilion depth and total depth are all shares of it.","on_report":"Not printed as its own line: average the first two numbers of the measurements line.","related":["measurements","total-depth","table-size","proportions"],"url":"https://digitalgemology.com/learn/glossary/average-diameter","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained"]},{"term":"Bearded girdle","slug":"bearded-girdle","aka":["bearding","beard"],"category":"Clarity characteristics","definition":"A bearded girdle has very small feathers that run from the girdle surface into the stone. They can come from the cutting process.","detail":"Fine, hairlike bearding is a clue to diamond. In GIA's 1997 study, moissanite girdles were frosted, polished or striated, without bearding.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["girdle","feather","rough-girdle","inclusion"],"url":"https://digitalgemology.com/learn/glossary/bearded-girdle","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Beer–Lambert law","slug":"beer-lambert-law","aka":["Beer's law"],"category":"Optics","definition":"The Beer–Lambert law says that the light a stone lets through falls off exponentially with path length: T = e^(−σℓ). Here σ is the absorption coefficient and ℓ is the path length.","detail":"A longer path deepens every absorption. So a large or deep stone looks darker than a thin slab of the same material.","how_we_measure":"The simulator applies it per color channel along each internal path. The Dichroscope, Spectroscope and Polariscope apply it per wavelength along one straight path.","related":["absorption-spectrum","pleochroism","3d-gemstone-simulator"],"url":"https://digitalgemology.com/learn/glossary/beer-lambert-law","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/about/how-we-measure/spectroscope"]},{"term":"Belly","slug":"belly","aka":["wings"],"category":"Anatomy and proportions","definition":"The belly is the wide middle of an elongated stone such as an oval, pear or marquise.","detail":"A bow-tie shows as a dark band across the belly. The belly facets start close to the center, so they must slope more steeply than the facets toward the ends. Their angles decide whether a band of leakage forms.","related":["bow-tie","head","shoulder","keel"],"url":"https://digitalgemology.com/learn/glossary/belly","sources":["https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds","https://digitalgemology.com/learn/bow-tie/which-diamond-shapes-have-a-bow-tie","https://digitalgemology.com/learn/bow-tie/how-to-fix-a-bow-tie"]},{"term":"Beryl","slug":"beryl","category":"Colored gemstones","definition":"Beryl is the mineral species of emerald. Aquamarine and the other beryl colors belong to the same species.","detail":"On our gemstone pages, emerald has a refractive index of 1.577 to 1.583 and a specific gravity of 2.72. It is 7.5 to 8 on the Mohs scale, and its crystal system is hexagonal. These values vary a little from deposit to deposit. Not every green beryl is emerald.","on_report":"LMHC wording prints '(natural) beryl' as the identification and 'emerald' as the species.","related":["emerald","green-beryl","gem-species","gem-variety","crystal-system"],"url":"https://digitalgemology.com/learn/glossary/beryl","sources":["https://digitalgemology.com/gemstones/emerald","https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Beryllium diffusion","slug":"beryllium-diffusion","aka":["lattice diffusion","Be diffusion","surface diffusion","bulk diffusion"],"category":"Treatments and origin","definition":"Beryllium diffusion is heating corundum with an added element, such as beryllium. The element diffuses into the crystal and changes its color.","detail":"It appeared on the market in the early 2000s and can strongly change the color of ruby and sapphire. In many stones it is extremely difficult to detect with certainty. If the new color is shallow, recutting can remove it. The LMHC wording also covers what labs once called 'surface diffusion'.","on_report":"Further information: indications of heating, color modified by (lattice) diffusion of a chemical element from an external source. Labs may add 'shallow' and name the element.","related":["heat-treatment","treatment","padparadscha","disclosure","sapphire"],"url":"https://digitalgemology.com/learn/glossary/beryllium-diffusion","sources":[]},{"term":"Bezel facet","slug":"bezel-facet","aka":["bezels","crown mains","kite facets","upper main facets"],"category":"Anatomy and proportions","definition":"Bezel facets are the 8 large four-sided facets on the crown of a round brilliant. They run from the table to the girdle.","detail":"The crown angle is measured on the bezels. Their slope bends the outgoing light and sets the crown height.","on_report":"Their slope is the crown angle on the proportion diagram.","related":["crown-angle","star-facet","upper-half-facet","crown"],"url":"https://digitalgemology.com/learn/glossary/bezel-facet","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-crown-angle","https://digitalgemology.com/learn/historic-cuts/peruzzi-cut-diamond"]},{"term":"Biaxial","slug":"biaxial","category":"Optics","definition":"A biaxial stone is a doubly refractive stone with two optic axes. It can show three colors in a dichroscope, and its interference figure differs from a uniaxial one.","detail":"Tanzanite and iolite are biaxial. On a refractometer, both shadow edges of a biaxial stone can move as the stone turns.","how_we_measure":"None of the seven gemstones in our tools is biaxial, so the tools do not model biaxial stones.","related":["uniaxial","optic-axis","pleochroism","interference-figure"],"url":"https://digitalgemology.com/learn/glossary/biaxial","sources":["https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Birefringence","slug":"birefringence","aka":["double refraction value"],"category":"Optics","definition":"Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.","detail":"Moissanite's birefringence is 0.043, amethyst's 0.009, ruby's and sapphire's 0.008 and emerald's 0.006. Diamond and cubic zirconia have none. On a refractometer, the largest gap between the two shadow edges gives it.","how_we_measure":"The Refractometer shows two edges for stones within its range. The Polariscope uses it to compute how a stone lights up between crossed polarizers. The simulator's render traces both indices, but every score uses a single index.","related":["doubly-refractive","ordinary-ray","extraordinary-ray","refractometer","polariscope"],"url":"https://digitalgemology.com/learn/glossary/birefringence","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/about/how-we-measure/refractometer","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Blemish","slug":"blemish","aka":["blemishes","external characteristic","surface characteristic"],"category":"Clarity characteristics","definition":"A blemish is a clarity characteristic confined to the surface of a diamond.","detail":"Polish lines, scratches, nicks, pits, abrasions, naturals and extra facets are blemishes. They can count in the polish grade as well as in clarity. An Internally Flawless stone has blemishes only.","on_report":"On GIA's plot, naturals are green and extra facets black. Many minor blemishes are not plotted; Comments can mention them.","related":["inclusion","internally-flawless","polish","natural","extra-facet"],"url":"https://digitalgemology.com/learn/glossary/blemish","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Blink (polariscope)","slug":"blink-polariscope","aka":["blinks","extinction positions"],"category":"Gem testing","definition":"A stone blinks in a polariscope when it turns light and dark four times per turn between crossed polarizers. Blinking shows that the stone is doubly refractive.","detail":"The dark positions come every 90°, where the stone's vibration directions line up with the filters. Looked at down its optic axis, a doubly refractive stone does not blink. This “blink” is not the blink ray class of our light census.","how_we_measure":"Our Polariscope calls a stone blinking when its darkest position over a full turn is under half its lightest. It says a stone “stays dark” when its lightest position stays under Y = 0.003. These thresholds are our own choice.","related":["polariscope","doubly-refractive","strain","optic-axis","census-ray-classes"],"url":"https://digitalgemology.com/learn/glossary/blink-polariscope","sources":["https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/tools/polariscope"]},{"term":"Bow-tie","slug":"bow-tie","aka":["bow-tie effect"],"category":"Cuts and shapes","definition":"A bow-tie is a dark band across the belly of an oval, pear, marquise or other elongated stone. It is not an inclusion or a color: the facets there send little light back to the viewer.","detail":"GIA's guides call it a reflection of the viewer's head and shoulders. They add that the shape and facet pattern play a part. In DG's ray trace of a diamond, belly facets over about 51.9° let head-on light leak out through the back. That leakage makes a dark band too, and the room light decides how dark either kind looks.","on_report":"A GIA report gives fancy shapes no cut grade and says nothing about a bow-tie. IGI's optional fancy-shape cut grade checks for bow-ties, and the AGS Ideal Report's ASET map covers some ovals.","related":["belly","fancy-shape","length-to-width-ratio","aset-map","leakage","contrast"],"url":"https://digitalgemology.com/learn/glossary/bow-tie","sources":["https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds","https://digitalgemology.com/learn/bow-tie/what-causes-a-bow-tie","https://digitalgemology.com/learn/bow-tie/bow-tie-tilt-and-head-shadow","https://digitalgemology.com/learn/bow-tie/how-to-fix-a-bow-tie","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report","https://digitalgemology.com/learn/bow-tie/how-to-check-a-diamond-for-a-bow-tie","https://digitalgemology.com/learn/bow-tie"]},{"term":"Brightness & Tilt Analysis","slug":"brightness-and-tilt-analysis","aka":["B&T","BTA"],"category":"How we measure","definition":"The Brightness & Tilt Analysis is a free Digital Gemology tool. It measures how much dome light a cut returns face-up, and how this changes with tilt.","detail":"It reads one trace four ways: Light Return and three dome models, ISO, COS and SC2. It works on 22 of our cuts and on your own file. Its figures describe the geometry, not a real stone.","how_we_measure":"Rays start on a 132 × 132 grid, straight down. Schlick's Fresnel approximation applies at entry and exit, and internal reflections lose nothing, for up to 12 bounces. The tilt chart traces 13 tilts from −30° to 30°, in 5° steps, on an 84 × 84 grid. You set the head shadow from 0° to 30°; the default is 10°.","related":["dome-model","iso-lighting","head-shadow","tilt-light-performance","schlick-approximation","leakage"],"url":"https://digitalgemology.com/learn/glossary/brightness-and-tilt-analysis","sources":["https://digitalgemology.com/tools/brightness-tilt-analysis","https://digitalgemology.com/about/how-we-measure/brightness-tilt"]},{"term":"Brilliance","slug":"brilliance","aka":["white light return"],"category":"Light performance","definition":"Brilliance is the white light that a cut stone returns to the viewer. In our census, it is the share of entering light that leaves through the crown toward the viewer.","detail":"Brilliance depends on the angles of the cut and on the refractive index of the stone. Our tools count it in different ways, so their figures are not interchangeable. The T73 in diamond reads 93 in our census and 54% in the Cut Grading Tool. Compare census brilliance only between cuts in the same gemstone.","how_we_measure":"The light census traces 3,200 rays per gemstone. Brilliance is 100 × the useful rays ÷ the rays that enter. A useful ray leaves through the crown within 64° of vertical, after at least one pavilion reflection. The Cut Grading Tool's brilliance is the ISO figure with a 23° head shadow, averaged over 13 tilts.","related":["fire","scintillation","light-census","census-ray-classes","cut-grading-tool","light-performance"],"url":"https://digitalgemology.com/learn/glossary/brilliance","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/about/how-we-measure/light-measures","https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/cut-grading-tool"]},{"term":"Brilliant cut","slug":"brilliant-cut","aka":["brilliant","brilliant cutting style"],"category":"Cuts and shapes","definition":"A brilliant cut is a cutting style with a crown of facets above a pavilion. Its facet angles return light and spread it into fire.","detail":"Light enters through the top, reflects off the pavilion and leaves at a different angle, so the colors separate. A rose cut, with no pavilion, can't do that. The first brilliants were the Mazarins of the mid-1600s. Many fancy shapes, such as the oval, pear and marquise, use a brilliant facet plan.","on_report":"Named in the shape and cutting style line, for example \"Oval Brilliant\".","related":["round-brilliant","step-cut","rose-cut","mazarin-cut","fire"],"url":"https://digitalgemology.com/learn/glossary/brilliant-cut","sources":["https://digitalgemology.com/learn/historic-cuts/mazarin-cut-diamond","https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section"]},{"term":"Bruise","slug":"bruise","aka":["bruises"],"category":"Clarity characteristics","definition":"A bruise is a tiny area of impact damage with very small, root-like feathers. It usually sits at a facet junction.","detail":"A bruise comes from a knock, not from the growth of the crystal. GIA lists it among the inclusions.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["chip","feather","inclusion"],"url":"https://digitalgemology.com/learn/glossary/bruise","sources":[]},{"term":"Bruting","slug":"bruting","aka":["girdling","bruting machine"],"category":"Cuts and shapes","definition":"Bruting is shaping the outline of a diamond, usually round, by grinding it against another diamond.","detail":"By hand, nobody could make a true circle, so early brilliants were cushion-shaped. Mechanical bruting in the 1870s made diamonds truly round. In Boston, Charles Field, foreman for Henry Morse, built one such machine. A bruted girdle keeps the surface from this process.","on_report":"The proportion diagram can describe a girdle as bruted.","related":["girdle","old-european-cut","old-mine-cut","round-brilliant"],"url":"https://digitalgemology.com/learn/glossary/bruting","sources":["https://digitalgemology.com/learn/historic-cuts/old-european-cut-diamond","https://digitalgemology.com/learn/historic-cuts/mazarin-cut-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness"]},{"term":"Burn mark","slug":"burn-mark","aka":["burned facet","burn"],"category":"Clarity characteristics","definition":"A burn mark is a hazy area on a facet surface, caused by too much heat during polishing. It is also called a burned facet.","detail":"It is a blemish. The polish grader looks for burn marks at 10x, with pits, nicks, polish lines and scratches.","on_report":"A blemish; it can count in the polish grade.","related":["polish","polish-lines","blemish"],"url":"https://digitalgemology.com/learn/glossary/burn-mark","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence"]},{"term":"Carat weight","slug":"carat-weight","aka":["carat","ct","points","pointer"],"category":"Four Cs","definition":"Carat weight is the weight of a gemstone. One carat is 200 milligrams, a fifth of a gram, and it has 100 points.","detail":"Carat is weight, not size. Width grows only with the cube root of weight. So a 2 ct diamond is only about 26% wider than a 1 ct. In our simulator, a 1.00 ct T73 diamond is 6.48 mm across and a 2.00 ct is 8.16 mm.","on_report":"In the grades block, to the hundredth of a carat. GIA's balance reads to five decimal places and IGI's to four; both report two.","related":["magic-sizes","measurements","four-cs","weight-ratio","specific-gravity"],"url":"https://digitalgemology.com/learn/glossary/carat-weight","sources":["https://digitalgemology.com/learn/four-cs/carat-weight-vs-size","https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section"]},{"term":"Cavity","slug":"cavity","aka":["cavities"],"category":"Clarity characteristics","definition":"A cavity is an angular opening in the surface of a diamond. It forms when part of a feather breaks away, or when a surface-reaching crystal comes out during polishing.","detail":"A cavity is an opening in the surface, left by a feather or a lost crystal. GIA lists it among the inclusions.","on_report":"Drawn in red and green together on the clarity plot of a full GIA report. A Dossier has no plot but can name it in its list of up to four.","related":["feather","knot","chip","inclusion"],"url":"https://digitalgemology.com/learn/glossary/cavity","sources":[]},{"term":"Census ray classes","slug":"census-ray-classes","aka":["useful ray","blink ray","grazing ray","lost ray"],"category":"How we measure","definition":"In our light census, every ray that enters a stone ends in one of five classes. The classes are useful, blink, leak, grazing and lost.","detail":"Useful: out through the crown within 64° of vertical, after at least one pavilion reflection. Blink: out through the crown without touching the pavilion. Leak: out through the pavilion, or through the crown heading down; grazing: out through the girdle. Lost: out of the viewing cone, or still inside after 12 bounces.","how_we_measure":"The exit facet and the exit direction set the class. Brilliance counts the useful rays, and fire counts the useful rays whose two colors leave more than 0.6° apart. The census “blink” class has nothing to do with blinking in a polariscope.","related":["light-census","brilliance","fire","leakage","blink-polariscope"],"url":"https://digitalgemology.com/learn/glossary/census-ray-classes","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"Chatoyancy","slug":"chatoyancy","aka":["cat's-eye effect","cat's eye"],"category":"Gemstone properties","definition":"Chatoyancy is a single band of light that glides across a gem, like the slit eye of a cat. Light reflects from many aligned, needle-like inclusions.","detail":"The stone must be cut as a cabochon to show it. Asterism is two or more chatoyant bands that cross.","on_report":"Labs name the phenomenon, for example cat's-eye chrysoberyl.","related":["asterism","needle","silk"],"url":"https://digitalgemology.com/learn/glossary/chatoyancy","sources":[]},{"term":"Chip","slug":"chip","aka":["chips"],"category":"Clarity characteristics","definition":"A chip is a shallow opening in the surface, caused by damage. It is usually at the girdle edge, a facet junction or the culet.","detail":"Thin girdles and pointed ends chip most easily. A culet facet exists to protect the point from chipping.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["girdle-thickness","culet","bruise","inclusion"],"url":"https://digitalgemology.com/learn/glossary/chip","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-culet","https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness"]},{"term":"CIE colorimetry","slug":"cie-colorimetry","aka":["CIE 1931 observer","illuminant D65","illuminant A"],"category":"Optics","definition":"CIE colorimetry turns a light spectrum into a color by weighting it with standard human color-matching functions. Standard illuminants, such as D65 (daylight) and A (incandescent lamp, 2856 K), define the light.","detail":"The CIE 1931 2° observer is the standard set of color-matching functions. A spectrum, an illuminant and the observer together give one reproducible color.","how_we_measure":"The Dichroscope and Polariscope use the CIE 1931 observer with D65, at 81 wavelengths from 380 to 780 nm. The Spectroscope lets you choose illuminant A or D65.","related":["ciede2000","dichroscope","spectroscope","absorption-spectrum"],"url":"https://digitalgemology.com/learn/glossary/cie-colorimetry","sources":["https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/about/how-we-measure/spectroscope","https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"CIEDE2000 color difference","slug":"ciede2000","aka":["ΔE00","delta E 2000"],"category":"Optics","definition":"CIEDE2000 (ΔE00) is a standard formula for how different two colors look to a human observer. A larger number means a larger visible difference.","detail":"It works in CIELAB lightness, chroma and hue, with corrections for how the eye sees each region. Our Dichroscope calls a ΔE00 under 1 “none”.","how_we_measure":"The Dichroscope uses ΔE00 to class pleochroism through 5 mm in daylight. Ruby reads 17.9 and sapphire 41.6 (strong), emerald 15.3 (distinct) and amethyst 3.3 (weak). We check our formula against the 34 test pairs of Sharma, Wu and Dalal (2005).","related":["pleochroism","dichroscope","cie-colorimetry"],"url":"https://digitalgemology.com/learn/glossary/ciede2000","sources":["https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/tools/dichroscope"]},{"term":"Citrine","slug":"citrine","aka":["yellow quartz"],"category":"Colored gemstones","definition":"Citrine is the pale yellow to brownish orange variety of quartz.","detail":"Natural citrine is rare. Most citrine on the market is heated amethyst. It has the same hardness and toughness as amethyst.","on_report":"Labs identify it as (natural) quartz, variety citrine.","related":["quartz","amethyst","ametrine","heat-treatment"],"url":"https://digitalgemology.com/learn/glossary/citrine","sources":[]},{"term":"Clarity characteristic","slug":"clarity-characteristic","aka":["clarity characteristics","flaw","imperfection"],"category":"Clarity characteristics","definition":"A clarity characteristic is any inclusion or blemish in or on a diamond. Together they set the clarity grade.","detail":"Their effect depends on their size, nature, number, position and relief at 10x. Like birthmarks, they identify one particular stone. They also help gemologists separate natural diamonds from synthetics and simulants.","on_report":"Drawn on the clarity plot and named in the key to symbols on a full report. A Dossier lists up to four in words.","related":["inclusion","blemish","clarity-plot","key-to-symbols","clarity-grade"],"url":"https://digitalgemology.com/learn/glossary/clarity-characteristic","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Clarity enhancement","slug":"clarity-enhancement","aka":["fissure filling","filling"],"category":"Treatments and origin","definition":"Clarity enhancement is filling surface-reaching fissures with a filler such as oil, resin, wax or glass, so they show less.","detail":"It is most common in emerald. Organic fillers can dry out or deteriorate, and a jeweler can remove or refill them. So labs state the condition only at the time of examination.","on_report":"Fissure filling, or indications of clarity enhancement/modification, with the amount: minor (F1), moderate (F2) or significant (F3).","related":["emerald-oiling","resin-filling","lead-glass-filling","enhancement-degree","fissure"],"url":"https://digitalgemology.com/learn/glossary/clarity-enhancement","sources":[]},{"term":"Clarity grade","slug":"clarity-grade","aka":["diamond clarity","clarity","clarity scale"],"category":"Four Cs","definition":"The clarity grade says how free a diamond is of inclusions (inside) and blemishes (on the surface) at 10x magnification. GIA's scale has 11 grades in six categories, from Flawless (FL) to Included (I3).","detail":"The grade is not a count. GIA bases it on the size, nature, number, position and relief of the characteristics. One dark crystal under the table can matter more than three faint pinpoints near the girdle. Many inclusions that graders find at 10x are not visible to the unaided eye.","on_report":"In the grades block, for example VS2. A full GIA report supports the grade with the clarity plot; a Dossier lists up to four characteristics in words.","related":["flawless","internally-flawless","vvs","vs","si","included"],"url":"https://digitalgemology.com/learn/glossary/clarity-grade","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Clarity plot","slug":"clarity-plot","aka":["plotted diagram","plotting diagram","inclusion map","clarity diagram"],"category":"Grading report","definition":"The clarity plot is the map on a full grading report: an outline of the stone with symbols on it. Each symbol shows the type, position and approximate size of a clarity characteristic.","detail":"The plot records the evidence for the clarity grade and fingerprints the stone. On GIA's plot, red shows most inclusions, green shows naturals and black shows extra facets. Graders leave out characteristics that are widespread or faint; Comments can mention them.","on_report":"On GIA's Diamond Grading Report, with the key to symbols beside it. The Diamond Dossier has no plot. IGI also maps the stone, segment by segment.","related":["key-to-symbols","clarity-characteristic","inclusion","blemish","diamond-dossier","clarity-grade"],"url":"https://digitalgemology.com/learn/glossary/clarity-plot","sources":["https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot","https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Cleavage","slug":"cleavage","aka":["cleavage plane","cleavage direction"],"category":"Gemstone properties","definition":"Cleavage is a mineral's tendency to split along flat planes in its crystal structure. A hard knock in a cleavage direction can split a stone.","detail":"Diamond has cleavage, so it can split even though it is the hardest gem. Corundum has no cleavage. GIA's researchers broke a moissanite cube with a hammer: it showed curved, shell-like fractures and no cleavage.","related":["toughness","hardness","fracture","durability","crystal-system"],"url":"https://digitalgemology.com/learn/glossary/cleavage","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care"]},{"term":"Cloud","slug":"cloud","aka":["clouds"],"category":"Clarity characteristics","definition":"A cloud is many tightly grouped pinpoints that are too small to see one by one but together look hazy.","detail":"A cloud is an inclusion. A large cloud can lower the transparency of a stone.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["pinpoint","twinning-wisp","included","inclusion"],"url":"https://digitalgemology.com/learn/glossary/cloud","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Coating","slug":"coating","aka":["coated diamond","surface coating"],"category":"Treatments and origin","definition":"Coating is a thin film, paint or ink on the surface of a gem that changes its color or look.","detail":"On a diamond, a very thin layer can mask the body color. Coatings can scratch and wear off, and once a gemologist suspects one, it is easy to identify. GIA treats coating as unstable and issues no grading report for a coated diamond. A coated stone cannot be called padparadscha.","on_report":"Labs state the coating. For a coated diamond, GIA issues an Identification Report instead of a grading report.","related":["color-treatment","fracture-filling","identification-report","treatment","disclosure","padparadscha"],"url":"https://digitalgemology.com/learn/glossary/coating","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin"]},{"term":"Color change","slug":"color-change","aka":["color-change sapphire","color-change effect"],"category":"Gemstone properties","definition":"Color change is a visible change of a gem's main hue between daylight and incandescent light.","detail":"Labs compare daylight of 5500 K to 6500 K with incandescent light of 2700 K to 3600 K. Color-change sapphires often go from blue in daylight to purple in incandescent light. Alexandrite, a chromium-bearing chrysoberyl, also changes color. Pleochroism differs: it changes with the direction of view, not the light.","on_report":"Color: [color] in daylight, [color] in incandescent light. Species: color-change [variety].","related":["sapphire","hue","pleochroism","fancy-sapphire"],"url":"https://digitalgemology.com/learn/glossary/color-change","sources":[]},{"term":"Color grade","slug":"color-grade","aka":["diamond color","D-to-Z scale","D to Z color scale","color"],"category":"Four Cs","definition":"The color grade is a letter from D to Z for the yellow or brown tint of a near-colorless diamond. D is colorless and Z is light yellow or brown.","detail":"Its 23 letters fall into five ranges: colorless, near colorless, faint, very light and light. GIA started the scale at D to break with the vague A, B and C grades of the past. Labs grade color against master diamonds in standard light, because warm or dim light hides small steps. A diamond with more color than Z leaves the scale and can get a fancy color grade.","on_report":"In the grades block. An asterisk after the letter means a detected color treatment. IGI also grades D to Z and groups the letters into named bands.","related":["master-stones","colorless","near-colorless","fancy-color-diamond","asterisk-color-grade","fluorescence"],"url":"https://digitalgemology.com/learn/glossary/color-grade","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z","https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-color-and-fluorescence"]},{"term":"Color treatment","slug":"color-treatment","aka":["treated color"],"category":"Treatments and origin","definition":"A color treatment is any process that changes a diamond's color, such as HPHT processing or coating.","detail":"A stable color treatment such as HPHT is graded and disclosed. Coating is unstable, so GIA doesn't grade a coated diamond.","on_report":"GIA adds an asterisk after the color grade and explains the treatment in Comments. It inscribes TREATED COLOR on an HPHT-treated stone.","related":["asterisk-color-grade","hpht","coating","treatment"],"url":"https://digitalgemology.com/learn/glossary/color-treatment","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/grading-reports/diamond-report-grades"]},{"term":"Color zoning","slug":"color-zoning","aka":["color banding","zoning"],"category":"Colored gemstones","definition":"Color zoning is uneven color in a gem: bands or patches of deeper and paler color.","detail":"It is common in sapphire and amethyst. To check, place the stone table-down on a white surface; zoning shows best through the pavilion. Cutters place a dark zone low in the pavilion, so the facets spread its color across the face.","related":["sapphire","amethyst","curved-striae","hue","pleochroism"],"url":"https://digitalgemology.com/learn/glossary/color-zoning","sources":["https://digitalgemology.com/gemstones/sapphire","https://digitalgemology.com/gemstones/amethyst","https://digitalgemology.com/gemstones/amethyst/asscher","https://digitalgemology.com/gemstones/sapphire/emerald-cut"]},{"term":"Colored stone report","slug":"colored-stone-report","aka":["colored stone identification report","gemstone report"],"category":"Treatments and origin","definition":"A colored stone report is a lab document that identifies a gem and states any treatment the lab detects. GIA's identification report describes the color but does not grade it.","detail":"GIA's identification reports list the item's weight, measurements, shape, color and cutting style. Then they state what the material is, with laboratory-grown material labeled as such. Comments follow. Some colored stone reports also give a geographic origin.","related":["origin-report","treatment","synthetic-gemstone","simulant","disclosure"],"url":"https://digitalgemology.com/learn/glossary/colored-stone-report","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report","https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report"]},{"term":"Colorless (D–F)","slug":"colorless","aka":["D-F","D color","E color","F color","colorless range"],"category":"Color and fluorescence","definition":"Colorless is the top range of the D-to-Z scale: the grades D, E and F.","detail":"These are the rarest grades. Even a trained grader can tell them apart only beside master stones.","on_report":"Shown by the color letter; IGI names its bands on the report.","related":["color-grade","near-colorless","master-stones"],"url":"https://digitalgemology.com/learn/glossary/colorless","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z"]},{"term":"Comments","slug":"comments","aka":["comments section","report comments"],"category":"Grading report","definition":"Comments is the report section for identifying characteristics or features that have no other line on the report. It is also where the lab describes a treatment that it detected.","detail":"GIA discloses both color and clarity treatments in Comments. If the color grade carries an asterisk, Comments explains it. GIA can also mention characteristics there that it didn't plot.","on_report":"Near the bottom of the report. At IGI, Comments notes the LG inscription on a lab-grown stone and permanent treatments. It can also give the growth method on request and the \"Ideal\" comment on a qualifying round.","related":["asterisk-color-grade","laser-drilling","hpht","ideal-cut","inscription"],"url":"https://digitalgemology.com/learn/glossary/comments","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/grading-reports/how-to-read-a-diamond-grading-report"]},{"term":"Conoscope","slug":"conoscope","aka":["conoscope sphere"],"category":"Gem testing","definition":"A conoscope is a strain-free glass sphere set on a stone in a polariscope. It gathers light from many directions at once, so the eyepiece shows an interference figure.","detail":"With the figure, a gemologist can tell a uniaxial stone from a biaxial one. With a first-order red plate, the figure also gives the optic sign.","how_we_measure":"Our Polariscope's conoscope maps a 40° cone of directions in air. The 40° is a working choice, not a measured sphere.","related":["interference-figure","red-plate","optic-sign","polariscope"],"url":"https://digitalgemology.com/learn/glossary/conoscope","sources":["https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Contact liquid","slug":"contact-liquid","aka":["RI liquid","refractometer liquid"],"category":"Gem testing","definition":"Contact liquid is the drop of high-index liquid between a stone and the glass of a refractometer. It fills the air gap so that light can pass from the glass into the stone.","detail":"The most common liquid has an index of 1.81. That sets the highest index the instrument can read. It also makes a faint line at 1.81 on every reading.","how_we_measure":"Our Refractometer uses 1.81 liquid only.","related":["refractometer","over-the-limit","shadow-edge"],"url":"https://digitalgemology.com/learn/glossary/contact-liquid","sources":["https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Contrast (Light Return)","slug":"contrast","aka":["head-shadow contrast","contrast (Light Return)"],"category":"Light performance","definition":"Contrast is the part of a face-up stone whose light comes from 75° or higher above the girdle plane. This zone lies within 15° of the line of sight, where the viewer's head is.","detail":"The light map shows contrast in blue. These areas reflect the viewer's head, so in real light they often look dark. In a well-cut round they form a crisp, even pattern that gives the stone sharpness. Contrast is not leakage: leakage areas look out of the back of the stone.","how_we_measure":"The simulator's light map counts light from 75° elevation and above as contrast. The Brightness & Tilt Analysis and the Cut Grading Tool use the same zone.","related":["light-return","light-map","head-shadow","leakage","bow-tie"],"url":"https://digitalgemology.com/learn/glossary/contrast","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/learn/bow-tie/what-causes-a-bow-tie"]},{"term":"Convex","slug":"convex","aka":["convex solid"],"category":"Cutting and files","definition":"A convex stone has no inward corners: a straight line between any two points inside it stays inside. Most cut stones are convex; a heart, with its notch, is not.","detail":"A convex stone is the space on the inner side of every facet plane. That makes it fast to trace. A ray enters at the last plane it crosses and leaves at the first plane it meets.","how_we_measure":"Our light tools trace each convex cut as an intersection of half-spaces, one per facet. The census does not score the heart. A file of your own must be convex to within 0.1% of its size, or 0.2% for STEP.","related":["facet","watertight","light-census","measure-your-own-stone"],"url":"https://digitalgemology.com/learn/glossary/convex","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/about/how-we-measure/your-own-file","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"Corundum","slug":"corundum","aka":["aluminum oxide"],"category":"Colored gemstones","definition":"Corundum is the mineral species made only of aluminum and oxygen. Its red variety is ruby, and all its other colors are sapphire.","detail":"All corundum shares one set of properties: refractive index 1.762 to 1.770, birefringence 0.008, hardness 9 and specific gravity 4.00. Its crystal system is trigonal. Corundum forms only where silicon is scarce, and silicon is common, so natural corundum is relatively uncommon. It has excellent toughness and no cleavage.","on_report":"LMHC wording prints '(natural) corundum' as the identification, and 'ruby' or 'sapphire' as the species or variety.","related":["ruby","sapphire","gem-species","gem-variety","hardness","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/corundum","sources":["https://digitalgemology.com/gemstones/ruby","https://digitalgemology.com/gemstones/sapphire","https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"COS lighting","slug":"cos-lighting","aka":["COS dome","cosine dome"],"category":"How we measure","definition":"COS is a dome model in which light is brightest overhead and fades to zero at the horizon. Each direction is weighted by the sine of its elevation.","detail":"It is hardest on shallow cuts, because they depend on low-angle light. The definition follows Robert W. Strickland's GemRay user guide; the implementation is our own.","how_we_measure":"In the Brightness & Tilt Analysis, W = sin e for elevation e. W is 0 inside the head shadow and below the horizon.","related":["dome-model","iso-lighting","sc2-lighting","brightness-and-tilt-analysis"],"url":"https://digitalgemology.com/learn/glossary/cos-lighting","sources":["https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/tools/brightness-tilt-analysis"]},{"term":"Critical angle","slug":"critical-angle","category":"Optics","definition":"The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc = arcsin(1/n).","detail":"A higher index gives a smaller critical angle. Diamond's is 24.4°, moissanite's 22.2°, cubic zirconia's 27.7°, ruby's and sapphire's 34.6°, emerald's 39.4° and amethyst's 40.4°. A refractometer uses a different critical angle: the one in its glass, against the stone.","how_we_measure":"Our gemstone pages give each stone's critical angle from the lower end of its refractive index. The head-on test on each cut page checks which pavilion facets lie at or above it.","related":["refractive-index","total-internal-reflection","guide-minimum-pavilion-angle","head-on-light","windowing"],"url":"https://digitalgemology.com/learn/glossary/critical-angle","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/gemstones/diamond","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Crown","slug":"crown","aka":["top","crown of a diamond"],"category":"Anatomy and proportions","definition":"The crown is the part of a cut stone above the girdle. On a standard round brilliant it has 33 facets: the table, 8 stars, 8 bezels and 16 upper halves.","detail":"Light enters mostly through the crown and leaves through it again. On the way out, the sloped crown facets bend each color by a different amount. That spreads white light into fire.","on_report":"Described on the proportion diagram by the crown angle and the crown height.","related":["table","crown-angle","crown-height","star-facet","bezel-facet","upper-half-facet"],"url":"https://digitalgemology.com/learn/glossary/crown","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-crown-angle"]},{"term":"Crown angle","slug":"crown-angle","aka":["crown angle (degrees)"],"category":"Anatomy and proportions","definition":"The crown angle is the slope of the bezel facets, measured against the plane of the girdle.","detail":"Tolkowsky calculated 34.5° in 1919. GIA's Excellent limit for the crown angle alone is 31.5° to 36.5°. GIA grades the crown and pavilion as a pair, so a shallower crown can balance a steeper pavilion.","on_report":"On the proportion diagram, in degrees; GIA reports it to the nearest 0.5°. IGI's Ideal band is 33 to 36°.","related":["crown-height","bezel-facet","pavilion-angle","table-size","fire"],"url":"https://digitalgemology.com/learn/glossary/crown-angle","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-crown-angle","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"Crown height","slug":"crown-height","category":"Anatomy and proportions","definition":"Crown height is the height of the crown, from the girdle to the table. It is a percentage of the average diameter.","detail":"It follows from the table and the crown angle: half of (100% − table) × the tangent of the crown angle. For the T73's 53% table and 34.5° crown, that is about 16%. GIA's Excellent limit for crown height alone is 12.5% to 17.0%.","on_report":"On the proportion diagram as a percentage. IGI's Ideal band is 13 to 16.5%.","related":["crown-angle","table-size","total-depth","crown"],"url":"https://digitalgemology.com/learn/glossary/crown-height","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-crown-angle","https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained"]},{"term":"Crystal","slug":"crystal-inclusion","aka":["included crystal","mineral crystal"],"category":"Clarity characteristics","definition":"A crystal is a mineral crystal contained in a diamond.","detail":"A dark crystal under the table can set the grade on its own. GIA found none of the mineral crystals typical of diamond in its 1997 moissanite samples.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["pinpoint","knot","needle","inclusion"],"url":"https://digitalgemology.com/learn/glossary/crystal-inclusion","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Crystal system","slug":"crystal-system","category":"Gemstone properties","definition":"A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.","detail":"Diamond and cubic zirconia are cubic, so they are singly refractive. Beryl and moissanite are hexagonal; corundum and quartz are trigonal. These stones are uniaxial, so they can show pleochroism and blink in a polariscope. Some references count trigonal as part of the hexagonal system.","how_we_measure":"Our gemstone pages give each stone's crystal system and show a unit cell of its structure.","related":["singly-refractive","uniaxial","biaxial","birefringence","pleochroism","cleavage"],"url":"https://digitalgemology.com/learn/glossary/crystal-system","sources":["https://digitalgemology.com/gemstones/diamond","https://digitalgemology.com/gemstones/moissanite","https://digitalgemology.com/gemstones/ruby","https://digitalgemology.com/gemstones/emerald","https://digitalgemology.com/gemstones/amethyst","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences"]},{"term":"Cubic zirconia","slug":"cubic-zirconia","aka":["CZ","cubic zirconium oxide"],"category":"Diamond and simulants","definition":"Cubic zirconia (CZ) is a man-made diamond simulant. GIA names it as the most common simulant today.","detail":"CZ is much heavier than diamond, with a specific gravity of about 5.8. So in our simulator, a 1.00 ct T73 is only 5.48 mm across in CZ, against 6.48 mm in diamond. A CZ sold by weight looks small for its carat.","on_report":"GIA doesn't grade CZ; it identifies it on an Identification Report.","related":["simulant","moissanite","specific-gravity","identification-report"],"url":"https://digitalgemology.com/learn/glossary/cubic-zirconia","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-carat-size","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin"]},{"term":"Culet","slug":"culet","aka":["culet facet","pointed culet","open culet"],"category":"Anatomy and proportions","definition":"The culet is the bottom point of the pavilion. It can also be a tiny facet, parallel to the table, that keeps the point from chipping.","detail":"Without a culet facet, the pavilion mains meet in a point, often called pointed. A large culet shows face up as a spot in the center of the pattern. The Mazarin of the 1600s had a wide, open culet.","on_report":"On the proportion diagram as a size word: see culet size.","related":["culet-size","pavilion","pavilion-main","mazarin-cut"],"url":"https://digitalgemology.com/learn/glossary/culet","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-culet","https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond"]},{"term":"Culet size","slug":"culet-size","aka":["culet grade","culet: none"],"category":"Anatomy and proportions","definition":"Culet size is the word that a report uses for the size of the culet. The eight words are None, Very Small, Small, Medium, Slightly Large, Large, Very Large and Extremely Large.","detail":"None means that there is no culet facet: the mains meet in a point. The smallest sizes are judged by eye at 10x. GIA's cut booklet allows Excellent with a culet of Small or less, and Very Good with Medium. In our Cut Grading Tool, which reads GIA's tables, Extremely Large drops a T73 to Poor.","on_report":"On the proportion diagram as a word.","related":["culet","cut-grade","proportion-diagram"],"url":"https://digitalgemology.com/learn/glossary/culet-size","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-culet","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"Curved striae","slug":"curved-striae","aka":["curved growth lines","curved color banding"],"category":"Clarity characteristics","definition":"Curved striae are curved growth lines or color bands in a flame-fusion synthetic. Natural corundum grows with straight, angular zoning instead.","detail":"With round or elongated gas bubbles, curved striae point to a flame-fusion stone. They can be hard to see in pale stones. Immersion or short-wave ultraviolet light helps a gemologist see them.","related":["flame-fusion","synthetic-gemstone","color-zoning","inclusion"],"url":"https://digitalgemology.com/learn/glossary/curved-striae","sources":[]},{"term":"Cushion cut","slug":"cushion-cut","aka":["cushion","cushion brilliant"],"category":"Cuts and shapes","definition":"A cushion cut is a square outline with rounded corners and a brilliant facet plan. It developed from the old mine cut.","detail":"Before mechanical bruting, nobody could make a diamond truly round, so early brilliants were cushions. Whether a cushion shows a bow-tie depends on its facet design, so published lists disagree.","on_report":"No GIA cut grade.","related":["old-mine-cut","fancy-shape","bow-tie","peruzzi-cut"],"url":"https://digitalgemology.com/learn/glossary/cushion-cut","sources":["https://digitalgemology.com/cuts/cushion","https://digitalgemology.com/learn/historic-cuts/mazarin-cut-diamond","https://digitalgemology.com/learn/bow-tie/which-diamond-shapes-have-a-bow-tie"]},{"term":"Cut grade","slug":"cut-grade","aka":["diamond cut","cut","cut quality","GIA cut grade"],"category":"Four Cs","definition":"The cut grade rates how well a diamond's proportions and finish work, from Excellent to Poor. GIA gives it only to standard round brilliants in the D-to-Z color range.","detail":"GIA's system went onto reports in January 2006. Seven components set the grade, and the lowest one decides. They are face-up appearance (how bright it looks, fire, scintillation), weight ratio, durability, polish and symmetry. For Excellent, each proportion has its own limits, and GIA also grades them in combination.","on_report":"In the grades block of a GIA report for a D-to-Z round brilliant; absent for fancy shapes and fancy colors. IGI grades round cut Excellent to Poor with one published range per proportion. It also offers an optional cut grade for fancy shapes.","related":["weight-ratio","polish","symmetry","ideal-cut","proportion-diagram","brilliance"],"url":"https://digitalgemology.com/learn/glossary/cut-grade","sources":["https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined","https://digitalgemology.com/learn/four-cs/diamond-cut-grade-excellent-vs-poor","https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"Cut Grading Tool","slug":"cut-grading-tool","aka":["cut grade estimator"],"category":"How we measure","definition":"The Cut Grading Tool is a free Digital Gemology tool that grades a round brilliant diamond's cut from its proportions. It reads GIA's published cut grade estimation tables; it is for education and does not give a GIA grade.","detail":"The tables hold 14,652 graded cells: 18 table sizes, each a grid of 37 crown by 22 pavilion angles. Table sizes run from 50% to 67%. The grade is the worst of the grid cell, the limits on the other proportions, and the finish grades. Digital Gemology is not affiliated with GIA.","how_we_measure":"We read the tables from the vector data of the published document, with no interpolation. Brilliance is traced on the T73; fire and scintillation are modeled, not traced. It shows 815 light maps: one per crown and pavilion cell at a 53% table, and the T73 as cut.","related":["cut-grade","brilliance","light-map","crown-angle","pavilion-angle","t73-benchmark"],"url":"https://digitalgemology.com/learn/glossary/cut-grading-tool","sources":["https://digitalgemology.com/tools/cut-grading-tool","https://digitalgemology.com/about/how-we-measure/cut-grading-tool"]},{"term":"Cut Performance Index","slug":"cut-performance-index","aka":["CPI","Digital Gemology Cut Performance Index"],"category":"How we measure","definition":"The Cut Performance Index is Digital Gemology's open table of light-performance scores for the cuts we sell, in seven gemstones. It gives each cut brilliance, fire, scintillation, tilt, symmetry, spread, shape-norm, leakage and an overall score.","detail":"Each gemstone has its own table, because the same cut handles light differently in each stone. Compare cuts within one gemstone only, never across stones. Ruby and sapphire share one refractive index in the engine, so their figures are the same. The scores describe the design of a cut, not a real stone.","how_we_measure":"Our light census computes the scores, with the same code as the Cut Performance Wheel and the cut pages. On 31 October 2026 it scored 37 of our 40 cuts. The heart and two cuts with no table are not scored yet, nor are the five Platonic solids. The data is free under CC BY 4.0, as CSV and JSON, and archived on Zenodo (DOI 10.5281/zenodo.23844870).","related":["light-census","overall-score","cut-performance-wheel","brilliance","fire","fixed-settings"],"url":"https://digitalgemology.com/learn/glossary/cut-performance-index","sources":["https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/performance-wheel"]},{"term":"Cut Performance Wheel","slug":"cut-performance-wheel","aka":["performance wheel"],"category":"How we measure","definition":"The Cut Performance Wheel is a free Digital Gemology tool. It draws eight light-performance scores of one cut, in one gemstone, on a wheel.","detail":"Its eight axes are Global, brilliance, fire, scintillation, tilt, symmetry, spread and shape-norm. It shows the cut against the T73 in the same gemstone, or against another cut you choose. It uses the same scores as the cut pages and the Cut Performance Index. It is a simulation, not a grading report.","how_we_measure":"The light census gives brilliance, fire, tilt and Global. Scintillation, symmetry, spread and shape-norm follow from the facets and proportions. The wheel can also score your own cutting file or 3D model, on your device.","related":["cut-performance-index","light-census","overall-score","t73-benchmark","brilliance"],"url":"https://digitalgemology.com/learn/glossary/cut-performance-wheel","sources":["https://digitalgemology.com/tools/cut-performance-wheel","https://digitalgemology.com/about/how-we-measure/performance-wheel"]},{"term":"Cutting file","slug":"cutting-file","aka":["faceting design","faceting file"],"category":"Cutting and files","definition":"A cutting file is a faceting design: every facet's angle and index, in cutting order, for a faceting machine. It tells a faceter how to cut the stone.","detail":"An .asc design records the refractive index it was drawn for. The same angles behave differently in a stone of another index. Each design names its index gear.","how_we_measure":"Our cutting files contain a GemCad .asc design, a tier table (.csv) and a printable faceting diagram (.html). Each cut page states how closely the file matches the 3D model.","related":["gemcad-asc","tier-table","faceting-diagram","3d-model","cutting-file-accuracy"],"url":"https://digitalgemology.com/learn/glossary/cutting-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/3d-models"]},{"term":"Cutting order","slug":"cutting-order","category":"Cutting and files","definition":"Cutting order is the sequence in which a faceter cuts the tiers of a design. Each new tier is cut to meet the tiers before it.","detail":"The design gives the order, because a tier cut too early has nothing to meet. A typical round starts with the pavilion, then turns over for the crown.","how_we_measure":"Our tier table lists every tier in cutting order.","related":["tier","tier-table","meetpoint","faceting-machine"],"url":"https://digitalgemology.com/learn/glossary/cutting-order","sources":["https://digitalgemology.com/about/files"]},{"term":"Cutting-file accuracy","slug":"cutting-file-accuracy","aka":["file accuracy"],"category":"How we measure","definition":"Cutting-file accuracy states how closely a cutting file's facet angles match the 3D model of the same cut. Each cut page gives the largest angle difference and the mean.","detail":"It compares facet directions only, not where facets meet. The page gives the girdle on its own. For the T73, every crown and pavilion facet is within 0.009° of the model, and girdle facets within 0.016°.","how_we_measure":"Each .asc facet becomes a normal from its angle and its index on the gear. We turn the model about its axis in quarter-index steps, in both handedness. We refine the best turn to 0.001°. We keep the alignment with the smallest area-weighted mean angle. Facets smaller than 0.02% of the largest are slivers, and we leave them out.","related":["cutting-file","3d-model","gemcad-asc","index-gear","facet"],"url":"https://digitalgemology.com/learn/glossary/cutting-file-accuracy","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"CVD","slug":"cvd","aka":["chemical vapor deposition","CVD diamond"],"category":"Treatments and origin","definition":"CVD, chemical vapor deposition, is one of the two usual methods for growing diamond in a laboratory. The other is HPHT.","detail":"A CVD-grown diamond is diamond, with essentially the same properties as a natural one. Only a gem lab's instruments can tell the two apart.","on_report":"GIA's Laboratory-Grown Diamond Report (2020 to 2025) stated CVD or HPHT. IGI can note CVD as the growth method in Comments on request.","related":["hpht","laboratory-grown-diamond","laboratory-grown-diamond-report"],"url":"https://digitalgemology.com/learn/glossary/cvd","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences"]},{"term":"Czochralski growth","slug":"czochralski-growth","aka":["crystal pulling","pulled synthetic"],"category":"Treatments and origin","definition":"Czochralski growth makes a synthetic crystal from a melt. A seed crystal touches the molten material and is slowly pulled up as the crystal grows.","detail":"The method dates from the early 1900s. It is used for synthetic alexandrite, chrysoberyl, corundum and garnet. Like flame fusion, it grows crystals from a melt.","on_report":"Labs report the stone as laboratory-grown (synthetic).","related":["synthetic-gemstone","flame-fusion","flux-growth","corundum"],"url":"https://digitalgemology.com/learn/glossary/czochralski-growth","sources":[]},{"term":"Diamond","slug":"diamond","category":"Diamond and simulants","definition":"Diamond is a single element, carbon, with every atom bonded to four neighbors in a cubic crystal. It is the hardest gem, Mohs 10.","detail":"Its refractive index is 2.417, its dispersion 0.044 and its specific gravity 3.52. It is singly refractive. It can be natural or laboratory-grown; both are diamond.","related":["natural-diamond","laboratory-grown-diamond","moissanite","simulant","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/diamond","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/gemstones/diamond"]},{"term":"Diamond Dossier","slug":"diamond-dossier","aka":["GIA Diamond Dossier","Dossier"],"category":"Grading report","definition":"The Diamond Dossier is a shorter GIA report for loose natural diamonds of 0.15 to 1.99 carats, D to Z. It gives the four Cs and a proportion diagram, but no clarity plot.","detail":"Instead of a plot, it lists in words up to four of the most significant clarity characteristics. GIA laser-inscribes the report number on the girdle of every Dossier stone. GIA doesn't issue it for synthetics, simulants, mounted stones, HPHT-processed or color-treated stones, or unstable treatments. Since January 2023 the Dossier has been digital only.","on_report":"A document that calls itself a GIA Dossier but shows a plotted diagram is not a real Dossier.","related":["diamond-grading-report","clarity-plot","inscription","key-to-symbols","hpht"],"url":"https://digitalgemology.com/learn/glossary/diamond-dossier","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot","https://digitalgemology.com/learn/grading-reports/how-to-read-a-diamond-grading-report","https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section"]},{"term":"Diamond grading report","slug":"diamond-grading-report","aka":["grading report","diamond certificate","diamond cert","lab report","GIA report","IGI report"],"category":"Grading report","definition":"A diamond grading report is an independent lab's record of what a stone is and how it grades. It is true for the day the lab examined the stone.","detail":"It records whether the stone is natural or laboratory-grown, any treatment, and its four Cs, measurements and proportions. It is not an appraisal: it says what the stone is, not what it is worth. GIA issues its Diamond Grading Report for loose natural diamonds of 0.15 carat and up. Check the inscription, the online record and the measurements to make sure the report belongs to the stone.","on_report":"GIA's Diamond Grading Report gives the four Cs, a plotted diagram of the clarity characteristics and a proportion diagram. Round brilliants in the D-to-Z range also get a cut grade. GIA grades loose stones only; IGI also reports on set stones, \"as mounting permits\".","related":["diamond-dossier","laboratory-grown-diamond-report","identification-report","appraisal","report-number","four-cs"],"url":"https://digitalgemology.com/learn/glossary/diamond-grading-report","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report","https://digitalgemology.com/learn/grading-reports/how-to-read-a-diamond-grading-report","https://digitalgemology.com/learn/grading-reports"]},{"term":"Dichroscope","slug":"dichroscope","aka":["calcite dichroscope","London dichroscope"],"category":"Gem testing","definition":"A dichroscope is a gem-testing instrument that shows a stone's two pleochroic colors side by side. In the calcite type, a calcite rhomb splits the light into two images of a small window.","detail":"The two images carry light that vibrates at right angles. Two different colors show that the stone is doubly refractive and pleochroic. Matching colors prove nothing: a singly refractive stone, a colorless one, or a view down the optic axis all match. A London dichroscope uses two polarizing filters instead of calcite.","how_we_measure":"Our Dichroscope computes the window colors from measured polarized absorption spectra, through 5 mm in daylight (D65). It is a teaching model, not a test of a real stone.","related":["pleochroism","ordinary-ray","extraordinary-ray","ciede2000"],"url":"https://digitalgemology.com/learn/glossary/dichroscope","sources":["https://digitalgemology.com/tools/dichroscope","https://digitalgemology.com/about/how-we-measure/dichroscope"]},{"term":"Direct light (Light Return)","slug":"direct-light","aka":["direct","direct light (Light Return)"],"category":"Light performance","definition":"Direct light is light that reaches the viewer from 45° to 75° above the girdle plane. Our light map shows it in red.","detail":"This is the bright light from above, outside the zone of the viewer's head. A well-cut round in diamond returns most of its face-up area as direct light.","how_we_measure":"The simulator's light map classes each face-up ray by the elevation its light came from. Rays from 45° to 75° count as direct.","related":["light-return","light-map","indirect-light","contrast","leakage"],"url":"https://digitalgemology.com/learn/glossary/direct-light","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds"]},{"term":"Disclosure","slug":"disclosure","aka":["treatment disclosure"],"category":"Treatments and origin","definition":"Disclosure is telling the buyer, before the sale, that a gem is treated, laboratory-grown or an imitation.","detail":"GIA calls treatment disclosure legally required for anyone who sells a gem. US FTC Guides require it if a treatment is not permanent, needs special care or greatly affects value. Trade groups such as AGTA, ICA and CIBJO set rules for their members. A lab report records what the lab found, but the seller must still disclose.","related":["treatment","synthetic-gemstone","simulant","origin-report"],"url":"https://digitalgemology.com/learn/glossary/disclosure","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin"]},{"term":"Dispersion","slug":"dispersion","category":"Optics","definition":"Dispersion is how differently a stone bends red and violet light. Gemologists give it as the index difference between the B line (686.7 nm) and the G line (430.8 nm).","detail":"Dispersion is the source of fire. Diamond's published dispersion is 0.044 and moissanite's 0.104, about 2.4 times as much. Cubic zirconia's is 0.060, ruby's and sapphire's 0.018, emerald's 0.014 and amethyst's 0.013.","how_we_measure":"The gemstone pages use these published values. The simulator spreads each exit ray into three colors, at n − 0.3D, n and n + 0.3D. The light census uses its own fire spread Δ, about 0.86 of the published dispersion. The Cut Performance Index page lists Δ as “dispersion”: 0.038 for diamond, 0.09 for moissanite.","related":["fire","fire-spread","refractive-index","spectroscope"],"url":"https://digitalgemology.com/learn/glossary/dispersion","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/learn/diamond-vs-moissanite/why-moissanite-has-more-fire","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties"]},{"term":"Dome model","slug":"dome-model","aka":["light model","dome lighting"],"category":"How we measure","definition":"A dome model is an idealized light source: a dome over the stone that weights light by its elevation. Our Brightness & Tilt Analysis uses three dome models: ISO, COS and SC2.","detail":"A dome model asks only where the light comes from, not what scene or color it has. ISO weights every elevation equally, and COS favors light from overhead. SC2 favors a ring of light near 45°. The definitions follow Robert W. Strickland's GemRay user guide; the implementation is our own.","how_we_measure":"We follow each path back from the eye to the direction its light came from. Its elevation e sets the weight: W = 1 (ISO), sin e (COS) or 2 sin e cos e (SC2). W is 0 below the horizon and inside the head shadow. The Cut Grading Tool's brilliance uses the ISO dome with a 23° head shadow.","related":["iso-lighting","cos-lighting","sc2-lighting","brightness-and-tilt-analysis","head-shadow","gemray"],"url":"https://digitalgemology.com/learn/glossary/dome-model","sources":["https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/tools/brightness-tilt-analysis","https://digitalgemology.com/about/how-we-measure/light-measures"]},{"term":"Doubly refractive","slug":"doubly-refractive","aka":["double refraction","anisotropic","DR"],"category":"Optics","definition":"A doubly refractive stone splits light into two rays that vibrate at right angles and travel at different speeds. Each ray has its own refractive index.","detail":"Moissanite, ruby, sapphire, emerald and amethyst are doubly refractive. In moissanite, the split is large enough to double the facet edges you see through the stone. Along an optic axis, light does not split.","how_we_measure":"The simulator's render traces both rays in doubly refractive stones. The Refractometer shows two edges where the stone is within range, and the Polariscope shows it blink.","related":["birefringence","optic-axis","ordinary-ray","extraordinary-ray","facet-doubling"],"url":"https://digitalgemology.com/learn/glossary/doubly-refractive","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Durability","slug":"durability","category":"Gemstone properties","definition":"Durability is a gem's ability to resist scratching, breaking, and damage from heat, light or chemicals. It has three parts: hardness, toughness and stability.","detail":"A gem can be strong in one part and weak in another. Diamond is the hardest gem, but it can cleave. Emerald is 7.5 to 8 on the Mohs scale, but its toughness is only fair to good. Corundum is hard, has excellent toughness and is stable in normal wear.","related":["hardness","toughness","stability","cleavage","mohs-scale"],"url":"https://digitalgemology.com/learn/glossary/durability","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care","https://digitalgemology.com/gemstones/sapphire"]},{"term":"Durability (cut grade)","slug":"durability-cut-grade","category":"Four Cs","definition":"Durability is the part of GIA's cut grade that checks for a risk of damage. The risk comes from a very thin girdle or thin points.","detail":"It is one of the two design components of the cut grade, with weight ratio. GIA does not count a shallow crown as a risk unless the girdle is also very thin. In our Cut Grading Tool, which reads GIA's tables, a Very Thin girdle limits a T73 to Very Good. An Extremely Thin girdle limits it to Good.","on_report":"Not printed on its own line: read it from the girdle description.","related":["girdle-thickness","weight-ratio","cut-grade","girdle"],"url":"https://digitalgemology.com/learn/glossary/durability-cut-grade","sources":["https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined","https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness"]},{"term":"Dyeing","slug":"dyeing","aka":["dyed"],"category":"Treatments and origin","definition":"Dyeing is adding a colored substance to a gem's fractures or pores to change its color.","detail":"It is used on ruby, emerald and quartz, among others. Solvents such as acetone or alcohol can remove many dyes, and sunlight can fade some. When a colored stone has a colored filler in its fissures, LMHC wording calls it a colored filler.","on_report":"'Dyed' when a colored filler creates the color of a near-colorless stone. Otherwise 'colored filler in fissures', with its amount.","related":["treatment","clarity-enhancement","stability","disclosure"],"url":"https://digitalgemology.com/learn/glossary/dyeing","sources":[]},{"term":"Electrical conductivity tester","slug":"electrical-conductivity-tester","aka":["moissanite tester","electrical tester"],"category":"Gem testing","definition":"An electrical conductivity tester checks whether a stone conducts electricity. Many moissanites conduct, and most near-colorless diamonds do not.","detail":"In GIA's 1997 study, 13 of 23 moissanites conducted. Some gray and blue diamonds conduct too: type IIb diamonds, colored by boron. So a “moissanite” reading on a blue or gray stone needs a laboratory. Combined testers pair a thermal reading with a second test.","related":["thermal-diamond-tester","screening-device"],"url":"https://digitalgemology.com/learn/glossary/electrical-conductivity-tester","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Emerald","slug":"emerald","category":"Colored gemstones","definition":"Emerald is the green to bluish green variety of the mineral beryl. Chromium, vanadium or both cause its color.","detail":"Most emeralds have fractures, and some estimates say 90 percent or more are fracture-filled. Emerald is 7.5 to 8 on the Mohs scale, with fair to good toughness. So cutters often give it a step cut with cropped corners. Heat, steam and ultrasonic cleaners can damage it.","on_report":"Labs identify it as (natural) beryl, species emerald. The report states any fissure filling, often with its amount: minor, moderate or significant.","related":["beryl","green-beryl","jardin","emerald-oiling","clarity-enhancement","three-phase-inclusion"],"url":"https://digitalgemology.com/learn/glossary/emerald","sources":["https://digitalgemology.com/gemstones/emerald","https://digitalgemology.com/gemstones/emerald/emerald-cut","https://digitalgemology.com/cuts/emerald-cut","https://digitalgemology.com/tools/dichroscope","https://digitalgemology.com/tools/spectroscope"]},{"term":"Emerald cut","slug":"emerald-cut","category":"Cuts and shapes","definition":"An emerald cut is a rectangular step cut with cropped corners and a long, open table.","detail":"Its step facets show color and clarity plainly. The cropped corners protect a brittle stone at its weakest points.","on_report":"No GIA cut grade.","related":["step-cut","radiant-cut","fancy-shape","asscher-cut"],"url":"https://digitalgemology.com/learn/glossary/emerald-cut","sources":["https://digitalgemology.com/cuts/emerald-cut"]},{"term":"Emerald oiling","slug":"emerald-oiling","aka":["oiled emerald","oiling"],"category":"Treatments and origin","definition":"Emerald oiling is filling an emerald's fissures with a colorless oil to make them less visible.","detail":"Oil can seep out or dry over time, and steam can make it sweat out. The fissures can then show again, and a jeweler can re-oil the stone. Clean oiled emeralds with warm, soapy water only.","on_report":"Fissure filling or clarity enhancement, with its degree. Some labs also name the filler as oil.","related":["clarity-enhancement","resin-filling","emerald","enhancement-degree","stability"],"url":"https://digitalgemology.com/learn/glossary/emerald-oiling","sources":[]},{"term":"Enhancement degree","slug":"enhancement-degree","aka":["minor, moderate, significant","F1, F2, F3"],"category":"Treatments and origin","definition":"The enhancement degree is the amount of filler a lab finds in a gem's fissures: minor, moderate or significant.","detail":"LMHC wording codes it F1, F2 and F3 for fissures, and C1 to C3 for filled cavities. Lead-glass filling in corundum uses the same F1 to F3 scale. The degree describes the stone at the time of examination, because fillers can change later.","on_report":"For example: 'indications of moderate clarity enhancement/modification', or F2.","related":["clarity-enhancement","emerald-oiling","lead-glass-filling","resin-filling"],"url":"https://digitalgemology.com/learn/glossary/enhancement-degree","sources":[]},{"term":"Etch channel","slug":"etch-channel","aka":["etch channels"],"category":"Clarity characteristics","definition":"An etch channel is a narrow channel in a diamond that opens at the surface. It forms where the diamond dissolved along a line defect (a dislocation) in its crystal.","detail":"GIA calls etch channels rare in natural diamonds, both type I and type II. Their surface openings are rhombic, and some channels wander like worm-like ribbons. GIA graded a 1.77 ct type IIa diamond with a network of etch channels I1.","on_report":"Drawn in red and green together on the clarity plot of a full GIA report. A Dossier has no plot but can name it in its list of up to four.","related":["cavity","inclusion","natural-diamond","key-to-symbols"],"url":"https://digitalgemology.com/learn/glossary/etch-channel","sources":[]},{"term":"Extra facet","slug":"extra-facet","aka":["extra facets"],"category":"Clarity characteristics","definition":"An extra facet is a facet that the cutting style doesn't need, placed without regard for the stone's symmetry. It is most often near the girdle.","detail":"It is a blemish, and it also counts as a symmetry fault. A small bottom facet that isn't parallel to the table counts as an extra facet, not as a culet.","on_report":"Drawn in black on GIA's clarity plot and listed last in the key to symbols.","related":["blemish","culet","symmetry","natural"],"url":"https://digitalgemology.com/learn/glossary/extra-facet","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-culet","https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence"]},{"term":"Extraordinary ray","slug":"extraordinary-ray","aka":["e-ray"],"category":"Optics","definition":"In a uniaxial stone, the extraordinary ray (e-ray) vibrates in the plane of the optic axis and the light path. Its refractive index changes with direction, from ω along the axis to ε across it.","detail":"The extraordinary index ε is 2.691 in moissanite, 1.762 in ruby and sapphire, 1.577 in emerald and 1.553 in amethyst. Along the optic axis, the e-ray has the same index as the o-ray, so there is no split.","how_we_measure":"The simulator traces the e-ray with 1/n(θ)² = cos²θ/nₒ² + sin²θ/nₑ². Our Dichroscope shows its color: orangy red in ruby, bluish green in emerald.","related":["ordinary-ray","birefringence","optic-axis","pleochroism"],"url":"https://digitalgemology.com/learn/glossary/extraordinary-ray","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/about/how-we-measure/dichroscope"]},{"term":"Face-up size","slug":"face-up-size","aka":["face-up size per carat","spread (trade)"],"category":"How we measure","definition":"Face-up size is how large a stone looks from above for its weight. Two stones of the same carat weight can differ in face-up size because of their depth and shape.","detail":"A deep stone keeps more weight below the girdle, where it adds nothing to the face-up outline. A less dense material also faces up larger at the same weight. A 1.00 ct T73 is 6.48 mm across in diamond and 6.67 mm in moissanite.","how_we_measure":"Our gemstone cut pages measure face-up size per carat from each model's outline. The Spread score on the Cut Performance Wheel is a separate estimate, from depth alone.","related":["spread-score","specific-gravity","carat-weight","total-depth"],"url":"https://digitalgemology.com/learn/glossary/face-up-size","sources":["https://digitalgemology.com/gemstones/diamond/t73-round-brilliant","https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-carat-size"]},{"term":"Facet","slug":"facet","aka":["face","facets"],"category":"Cutting and files","definition":"A facet is one flat, polished face of a cut gemstone. Its angle and its direction around the stone decide where it sends light.","detail":"Facets are grouped into the crown above the girdle, the girdle, and the pavilion below it. In a faceting design, each facet has an angle and an index position on the gear. We measure facet angles from the girdle plane.","how_we_measure":"Our models are closed solids of flat facets. The tools trace each convex cut as the intersection of one half-space per facet.","related":["planar-facet","tier","facet-count","index-gear","crown","pavilion"],"url":"https://digitalgemology.com/learn/glossary/facet","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/about/files"]},{"term":"Facet count","slug":"facet-count","category":"Cutting and files","definition":"The facet count is the number of facets on a cut. On our cut pages, it is the number of planar faces of the cut's STEP solid.","detail":"A cutting design can count differently. A turned girdle is one surface in a solid but many facets on the machine. Then the page gives both counts. The T73 round brilliant has 73 facets: 57 plus 16 girdle facets.","how_we_measure":"Our scintillation score uses the number of facets that reach the surface of the cut.","related":["facet","scintillation","step-file","turned-girdle"],"url":"https://digitalgemology.com/learn/glossary/facet-count","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"Facet doubling","slug":"facet-doubling","aka":["doubled facet edges","doubling","loupe test"],"category":"Gem testing","definition":"Facet doubling is the double image of facet edges seen through a strongly doubly refractive stone. It shows clearly in moissanite under a 10× loupe.","detail":"Look through a crown facet at an angle, not straight down through the table. Along the optic axis, nothing doubles. Doubling proves that a stone is not diamond. It does not prove moissanite, because other doubly refractive stones double too.","how_we_measure":"The 3D Gemstone Simulator traces both rays, so moissanite's edges double in its render at an angle.","related":["doubly-refractive","birefringence","optic-axis","3d-gemstone-simulator"],"url":"https://digitalgemology.com/learn/glossary/facet-doubling","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test","https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"Faceting diagram","slug":"faceting-diagram","aka":["cutting diagram"],"category":"Cutting and files","definition":"A faceting diagram is a printable drawing of a faceting design. A faceter prints it, fits the gear and cuts the stone tier by tier.","detail":"It shows the same design as the .asc file, on paper. You need no design software to read it.","how_we_measure":"Our cutting files include a faceting diagram as an .html file. It opens in any web browser, and you can print it or save it as a PDF.","related":["cutting-file","tier-table","gemcad-asc"],"url":"https://digitalgemology.com/learn/glossary/faceting-diagram","sources":["https://digitalgemology.com/about/files"]},{"term":"Faceting machine","slug":"faceting-machine","category":"Cutting and files","definition":"A faceting machine cuts and polishes a stone one facet at a time. It holds the stone on a dop at a set angle against a flat, spinning lap.","detail":"An index gear on the machine sets the direction of each facet around the stone. Faceters cut colored stones and synthetics this way. Diamond is cut with other equipment: it is sawn, bruted and polished on a scaife.","related":["index-gear","cutting-file","tier","cutting-order"],"url":"https://digitalgemology.com/learn/glossary/faceting-machine","sources":["https://digitalgemology.com/about/files"]},{"term":"Faint (K–M)","slug":"faint-color","aka":["K-M","faint color range"],"category":"Color and fluorescence","definition":"Faint is the D-to-Z range from K to M: a faint yellow or brown tint.","detail":"The tint shows most in a larger stone. In our simulator, the first tint that you can see face up falls in this range.","related":["near-colorless","very-light-color","color-grade"],"url":"https://digitalgemology.com/learn/glossary/faint-color","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z"]},{"term":"Fancy color diamond","slug":"fancy-color-diamond","aka":["fancy colored diamond","colored diamond","fancy color"],"category":"Color and fluorescence","definition":"A fancy color diamond is a colored diamond with enough color for a fancy grade. To GIA, any diamond outside the D-to-Z scale is a colored diamond.","detail":"Colored diamonds come in yellow and brown, and also red, pink, blue, green and other colors. On the D-to-Z scale, less color is rarer. Past Z it turns around, and stronger, purer fancy color is more valued.","on_report":"GIA grades it on a Colored Diamond Grading Report, by hue plus an intensity word. A fancy-color diamond gets no GIA cut grade, even as a round.","related":["fancy-color-grade","hue-tone-saturation","color-grade","type-iib-diamond"],"url":"https://digitalgemology.com/learn/glossary/fancy-color-diamond","sources":["https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence","https://digitalgemology.com/learn/grading-reports/diamond-report-grades"]},{"term":"Fancy color grade","slug":"fancy-color-grade","aka":["Fancy Light","Fancy","Fancy Intense","Fancy Vivid","Fancy Deep","Fancy Dark","intensity grade"],"category":"Color and fluorescence","definition":"A fancy color grade names a colored diamond by its hue plus one grade word, such as Fancy Intense Yellow. The word sums up the tone and saturation of the color.","detail":"GIA's colored diamond grades start with Faint, Very Light and Light. The fancy grades follow: Fancy Light, Fancy, Fancy Intense, Fancy Vivid, Fancy Dark and Fancy Deep. GIA grades red and purplish red diamonds only as Fancy, because their tone and saturation fall in a narrow range. Our simulator uses five of the words: Fancy Light, Fancy, Fancy Intense, Fancy Vivid and Fancy Deep.","on_report":"Replaces the D-to-Z letter on a GIA Colored Diamond Grading Report.","related":["fancy-color-diamond","hue-tone-saturation","color-grade"],"url":"https://digitalgemology.com/learn/glossary/fancy-color-grade","sources":["https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence"]},{"term":"Fancy sapphire","slug":"fancy-sapphire","aka":["fancy-color sapphire"],"category":"Colored gemstones","definition":"A fancy sapphire is a sapphire of any color other than blue, such as yellow, orange, pink, purple or green.","detail":"Red corundum is ruby, not a fancy sapphire. Discoveries in East Africa and Madagascar in the 1990s brought fancy sapphires wide recognition.","on_report":"Labs put the color before the word: for example, yellow sapphire or pink sapphire.","related":["sapphire","padparadscha","pink-sapphire","color-change","corundum"],"url":"https://digitalgemology.com/learn/glossary/fancy-sapphire","sources":[]},{"term":"Fancy shape","slug":"fancy-shape","aka":["fancy cut","fancy-shaped diamond"],"category":"Cuts and shapes","definition":"A fancy shape is any diamond outline other than round, such as oval, pear, marquise, cushion, princess or emerald.","detail":"GIA gives fancy shapes no cut grade. IGI offers an optional fancy-shape cut grade from Excellent to Poor. Polish and symmetry must be Very Good or better, and the proportions must fit IGI's ranges for the shape. Then IGI checks for shape problems such as bow-ties, and a gemologist judges the light return.","on_report":"Measurements read length × width × depth. No GIA cut grade line.","related":["round-brilliant","cut-grade","bow-tie","length-to-width-ratio","oval-cut"],"url":"https://digitalgemology.com/learn/glossary/fancy-shape","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/learn/four-cs/diamond-cut-grade-excellent-vs-poor","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report"]},{"term":"FBX file","slug":"fbx-file","aka":["FBX"],"category":"Cutting and files","definition":"FBX is a 3D file format for animation and game engines such as Unity and Unreal Engine.","detail":"Maya, 3ds Max and Blender also open it. It stores the stone as a triangle mesh.","how_we_measure":"Our 3D files include FBX. Our tools read the mesh of your own FBX file, but they do not apply its model transforms.","related":["mesh","3d-model"],"url":"https://digitalgemology.com/learn/glossary/fbx-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Feather","slug":"feather","aka":["feathers"],"category":"Clarity characteristics","definition":"A feather is the general trade term for a break in a gemstone. It often looks white and feathery.","detail":"A feather can reflect light and flash bright as the stone turns. A fracture filling can be put into a surface-reaching feather. GIA advises caution with ultrasonic and steam cleaning for stones with feathers.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["fracture-filling","bearded-girdle","bruise","inclusion"],"url":"https://digitalgemology.com/learn/glossary/feather","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care"]},{"term":"Fingerprint","slug":"fingerprint","aka":["healed fracture","healed fissure"],"category":"Clarity characteristics","definition":"A fingerprint is a pattern of tiny fluid or glassy drops left where a crack inside the crystal healed. It often looks like a human fingerprint.","detail":"Fingerprints are common in natural ruby and sapphire. Heating can heal fissures and leave glassy residues, which labs report. Flux-grown synthetics can also have flux fingerprints that look natural, so an expert must judge them.","on_report":"After heating, labs may describe residues in healed fissures as minor, moderate or significant.","related":["residues-in-healed-fissures","flux-growth","fissure","inclusion","heat-treatment"],"url":"https://digitalgemology.com/learn/glossary/fingerprint","sources":[]},{"term":"Fire","slug":"fire","aka":["dispersion flashes","spectral flashes"],"category":"Light performance","definition":"Fire is the flashes of spectral color that a cut stone sends out. In our census, fire is the share of returned light whose colors leave more than 0.6° apart.","detail":"Fire comes from dispersion: the stone bends violet light more than red light. Our fire is a share of the returned light, not of all light. So a dim cut can still score high. Fire shows most under small, bright light sources and fades under soft, even light.","how_we_measure":"The census refracts each useful ray out twice, at the index minus and plus the fire spread Δ. If the two directions differ by more than 0.6°, the ray counts as fire. Fire = 100 × fire rays ÷ useful rays. The Cut Grading Tool's fire is a smooth model of crown and pavilion angle, not a trace.","related":["dispersion","fire-spread","brilliance","light-census","cut-performance-wheel"],"url":"https://digitalgemology.com/learn/glossary/fire","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/learn/diamond-vs-moissanite/why-moissanite-has-more-fire","https://digitalgemology.com/about/how-we-measure/cut-grading-tool"]},{"term":"Fire spread (Δ)","slug":"fire-spread","aka":["census dispersion"],"category":"How we measure","definition":"Fire spread Δ is the index difference that our light census uses to split a ray into two colors. It is about 0.86 of each stone's published dispersion.","detail":"The census refracts each useful ray out at n − Δ and at n + Δ. Δ is 0.038 for diamond, 0.090 for moissanite, 0.052 for cubic zirconia and 0.015 for ruby and sapphire. It is 0.012 for emerald and 0.011 for amethyst.","how_we_measure":"Our research engine holds these constants for the light census. The gemstone pages and the simulator use the published dispersion, such as 0.044 for diamond and 0.104 for moissanite. The Brightness & Tilt Analysis uses no dispersion.","related":["dispersion","fire","light-census","refractive-index","cut-performance-index"],"url":"https://digitalgemology.com/learn/glossary/fire-spread","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"First-order red plate","slug":"red-plate","aka":["red plate","compensator","full-wave plate"],"category":"Gem testing","definition":"A first-order red plate is a retarder in a polariscope that shows which of a stone's two rays is slower. It turns parts of the interference figure blue or yellow, and that gives the optic sign.","detail":"Where the stone's slow ray lines up with the slow direction of the plate, the color climbs to blue. Where they cross, it falls to yellow.","how_we_measure":"Our Polariscope's red plate is a 550 nm linear retarder, with its slow direction from northeast to southwest. The page reads the sign from the figure's interference colors alone.","related":["optic-sign","conoscope","interference-figure","polariscope"],"url":"https://digitalgemology.com/learn/glossary/red-plate","sources":["https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Fish-eye","slug":"fish-eye","aka":["fisheye","fish-eye effect"],"category":"Cuts and shapes","definition":"A fish-eye is a dull ring that shows under the table of a round diamond. It is a reflection of the girdle, seen when the pavilion is too shallow.","detail":"A large table makes it worse, and it shows more as the stone tilts. GIA counts a fish-eye against the pattern in its cut grade. IGI's round cut chart caps the grade at Fair when a fish-eye is present.","on_report":"Not a line of its own. On an IGI round, a fish-eye limits the cut grade to Fair.","related":["pavilion-angle","table-size","cut-grade","windowing"],"url":"https://digitalgemology.com/learn/glossary/fish-eye","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-grades"]},{"term":"Fissure","slug":"fissure","aka":["fracture (clarity)","surface-reaching fracture"],"category":"Clarity characteristics","definition":"A fissure is a crack in a gem, often open to the surface. Labs use the word for the cracks that fillers can enter.","detail":"Almost all emeralds have fissures, and cutters plan around them. A surface-reaching fissure can weaken a stone. It also lets oil, resin or glass in, which makes it harder to see.","on_report":"Filled fissures are reported with the filler and its degree: minor, moderate or significant.","related":["clarity-enhancement","emerald-oiling","lead-glass-filling","fingerprint","toughness"],"url":"https://digitalgemology.com/learn/glossary/fissure","sources":[]},{"term":"Fixed settings","slug":"fixed-settings","aka":["method constants"],"category":"How we measure","definition":"Fixed settings are the constants of our scoring method that we chose rather than measured. They are the same for every cut, so the scores can rank cuts within one gemstone.","detail":"The census settings are a 6.9° light cone, a 64° viewing limit, a 0.6° fire threshold and a 20° tilt. The scoring settings are the overall weights, the scintillation constant 45 and the symmetry factor 6. They also include the 61.5% reference depth and the shape-family ceilings 93, 86, 76 and 64. Because of them, the scores rank cuts against each other; they are not physical constants.","how_we_measure":"The Cut Performance Index lists the fixed settings with the formulas that use them. The overall weights are 0.34, 0.20, 0.16, 0.10, 0.10 and 0.10.","related":["light-census","overall-score","cut-performance-index","shape-norm","spread-score"],"url":"https://digitalgemology.com/learn/glossary/fixed-settings","sources":["https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/performance-wheel"]},{"term":"Flame fusion","slug":"flame-fusion","aka":["Verneuil process","flame-fusion synthetic"],"category":"Treatments and origin","definition":"Flame fusion grows synthetic crystals by dropping fine powder through a very hot flame. The melted drops build up a crystal on a pedestal below.","detail":"Auguste Verneuil announced the process in 1902, so it is also called the Verneuil process. It is still the least expensive and most common way to make synthetic corundum and spinel. Its stones can show gas bubbles and curved growth lines, which natural corundum does not have.","on_report":"Labs report the stone as laboratory-grown (synthetic). They may not name the method.","related":["synthetic-gemstone","curved-striae","flux-growth","czochralski-growth","ruby"],"url":"https://digitalgemology.com/learn/glossary/flame-fusion","sources":[]},{"term":"Flawless (FL)","slug":"flawless","aka":["FL","Flawless"],"category":"Four Cs","definition":"Flawless (FL) is the top clarity grade: a skilled grader finds no inclusions and no blemishes at 10x magnification.","detail":"Flawless is strict. A few very small polish marks can still allow an Excellent polish grade. But those marks can stop the stone from being Flawless.","on_report":"Written \"Flawless\" or \"FL\" in the clarity grade line.","related":["internally-flawless","clarity-grade","blemish","10x-magnification"],"url":"https://digitalgemology.com/learn/glossary/flawless","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Fluorescence","slug":"fluorescence","aka":["UV fluorescence","blue fluorescence","inert"],"category":"Color and fluorescence","definition":"Fluorescence is visible light, almost always blue, that some diamonds give off while ultraviolet light shines on them. It stops when the UV goes off.","detail":"GIA reports that about 25 to 35% of the diamonds it sees fluoresce, more than 95% of them blue. Only about 10% of fluorescent diamonds are Medium or stronger, and fewer than 0.2% look hazy or oily. Most moissanite is inert under long-wave UV. So a blue glow points to diamond, but no glow proves nothing.","on_report":"A line in additional grading: strength under long-wave UV (None, Faint, Medium, Strong, Very Strong) and color. On GIA reports, \"None\" covers everything from indiscernible to very faint. IGI records strength and color when present.","related":["color-grade","moissanite","fancy-color-diamond","diamond-grading-report"],"url":"https://digitalgemology.com/learn/glossary/fluorescence","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence","https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-color-and-fluorescence"]},{"term":"Flux growth","slug":"flux-growth","aka":["flux-grown"],"category":"Treatments and origin","definition":"Flux growth is a way to grow synthetic crystals from a chemical solvent (flux). Crystals form as the hot solution slowly cools.","detail":"It is used for synthetic emerald, ruby, sapphire, alexandrite and spinel. Growth can take up to a year. Flux remains can form fingerprint-like patterns that look like natural ones. Platinum platelets from the crucible show that a stone is synthetic.","on_report":"Labs report the stone as laboratory-grown (synthetic).","related":["synthetic-gemstone","flame-fusion","hydrothermal-growth","fingerprint","emerald"],"url":"https://digitalgemology.com/learn/glossary/flux-growth","sources":[]},{"term":"Four Cs","slug":"four-cs","aka":["4Cs","the 4 Cs","four Cs of diamonds"],"category":"Four Cs","definition":"The four Cs are color, clarity, cut and carat weight: four scales that describe the quality of a diamond. GIA made them into a grading system in the 1950s.","detail":"Color goes from D to Z, and clarity has 11 grades from FL to I3. Cut goes from Excellent to Poor, and GIA grades it for standard round brilliants only. One carat is 200 mg. No single C sets the value, but cut changes the look of the whole face.","on_report":"The grades block of every diamond report: carat weight, color grade, clarity grade and, where given, cut grade.","related":["carat-weight","color-grade","clarity-grade","cut-grade","diamond-grading-report"],"url":"https://digitalgemology.com/learn/glossary/four-cs","sources":["https://digitalgemology.com/learn/four-cs/the-four-cs-explained","https://digitalgemology.com/learn/four-cs/which-of-the-four-cs-matters-most","https://digitalgemology.com/learn/four-cs"]},{"term":"Fracture","slug":"fracture","aka":["conchoidal fracture"],"category":"Gemstone properties","definition":"Fracture is the way a gem breaks when it does not split along a cleavage plane. Conchoidal fracture leaves curved, shell-like surfaces.","detail":"Moissanite breaks with curved, shell-like fractures. A fracture is also the name for a crack inside a stone. In colored stones, labs often call such cracks fissures.","related":["cleavage","toughness","fissure","durability"],"url":"https://digitalgemology.com/learn/glossary/fracture","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care"]},{"term":"Fracture filling","slug":"fracture-filling","aka":["fracture-filled","clarity enhanced","glass filling"],"category":"Treatments and origin","definition":"Fracture filling is a clarity treatment that injects a glass-like filler into a crack to make it less visible.","detail":"It is unstable, so GIA issues no grading report for a fracture-filled diamond, only an Identification Report. Take care with ultrasonic and steam cleaning for a filled stone.","on_report":"No GIA grading report: an Identification Report instead. IGI also issues no full grading report for non-permanent treatments.","related":["feather","treatment","identification-report","coating"],"url":"https://digitalgemology.com/learn/glossary/fracture-filling","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care"]},{"term":"Fresnel reflection","slug":"fresnel-reflection","aka":["Fresnel equations","surface reflection"],"category":"Optics","definition":"Fresnel reflection is the share of light that a surface reflects instead of letting through. The Fresnel equations give that share from the two refractive indices, the angle and the polarization.","detail":"At the crown, part of the light reflects straight off and never enters. As light leaves the stone, part of it reflects back inside. A higher index reflects more: about 17% at normal incidence for diamond.","how_we_measure":"The simulator uses the full Fresnel equations, per polarization in doubly refractive stones. The Brightness & Tilt Analysis and the Cut Grading Tool use Schlick's approximation at entry and exit. The light census and the light map use none. The Refractometer uses the full equations for both polarizations to light its scale.","related":["schlick-approximation","total-internal-reflection","polarized-light","ray-tracing","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/fresnel-reflection","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/about/how-we-measure/light-measures","https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Gem Cut Studio","slug":"gem-cut-studio","aka":[".gcs"],"category":"Cutting and files","definition":"Gem Cut Studio is a faceting design and ray-tracing program for Windows and macOS. It opens GemCad .asc files directly, and its own design files end in .gcs.","detail":"It adds tools for tilt performance, light paths and yield. It can also export a design as OBJ or STL.","how_we_measure":"Our tools read your own .gcs files.","related":["gemcad-asc","cutting-file","measure-your-own-stone"],"url":"https://digitalgemology.com/learn/glossary/gem-cut-studio","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Gem species","slug":"gem-species","aka":["mineral species","species"],"category":"Colored gemstones","definition":"A gem species is a mineral with one chemical composition and one crystal structure, such as corundum, beryl or quartz.","detail":"The species sets the optical and physical properties: refractive index, hardness and specific gravity. Ruby and blue sapphire share the refractive indices of corundum, but not their color. A species can have many varieties.","on_report":"Report layouts differ. Some labs print the mineral as 'Identification' and the variety name, such as ruby, under 'Species'.","related":["gem-variety","corundum","beryl","quartz","crystal-system"],"url":"https://digitalgemology.com/learn/glossary/gem-species","sources":["https://digitalgemology.com/about/how-we-measure/dichroscope"]},{"term":"Gem variety","slug":"gem-variety","aka":["variety"],"category":"Colored gemstones","definition":"A variety is a subdivision of a gem species, usually by color. Ruby and sapphire are varieties of corundum, and emerald is a variety of beryl.","detail":"Varieties of one species share their optical and physical properties. Color belongs to the variety, so ruby and blue sapphire absorb light differently. The line between two varieties, such as ruby and pink sapphire, is a color judgment.","on_report":"The variety name is usually the name the buyer knows: ruby, sapphire, emerald, amethyst.","related":["gem-species","ruby","pink-sapphire","green-beryl","hue"],"url":"https://digitalgemology.com/learn/glossary/gem-variety","sources":["https://digitalgemology.com/about/how-we-measure/dichroscope"]},{"term":"GemCad .asc","slug":"gemcad-asc","aka":["GemCad",".asc file",".gem file"],"category":"Cutting and files","definition":"GemCad is Robert Strickland's faceting design program for Windows, and .asc is its text design format. An .asc file gives the gear, the refractive index and every facet's angle and index.","detail":"It is the standard format for sharing faceting designs. GemCad, GemRay and Gem Cut Studio open it. GemCad writes a vertical girdle as −89.99°.","how_we_measure":"Our cutting files include a GemCad .asc. Our tools also read your own .asc (text) and .gem (binary) GemCad files.","related":["cutting-file","gemray","gem-cut-studio","index-gear"],"url":"https://digitalgemology.com/learn/glossary/gemcad-asc","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"GemRay","slug":"gemray","category":"Cutting and files","definition":"GemRay is Robert Strickland's ray tracer for GemCad designs, for Windows. It shows how a finished stone will look and how it darkens as it tilts.","detail":"GemRay's user guide defines the ISO, COS and SC2 dome models. Faceters use it to adjust a design's angles before they cut.","how_we_measure":"Our Brightness & Tilt Analysis follows the GemRay definitions of ISO, COS and SC2, with our own implementation. We do not count partial internal reflections. So the dip near face-up is shallower than GemRay shows.","related":["gemcad-asc","dome-model","iso-lighting","brightness-and-tilt-analysis","gem-cut-studio"],"url":"https://digitalgemology.com/learn/glossary/gemray","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/brightness-tilt"]},{"term":"Girdle","slug":"girdle","aka":["girdle edge","faceted girdle","bruted girdle","polished girdle"],"category":"Anatomy and proportions","definition":"The girdle is the narrow band at the widest part of a cut stone, between the crown and the pavilion. It is the edge that a setting grips.","detail":"A thick enough girdle protects the edge from chips. It can be bruted, polished or faceted, and graders judge the thickness the same way for all three. The T73 has a girdle band of 16 facets; the T57 has a knife-edge. Labs also use the girdle for inscriptions.","on_report":"On the proportion diagram as a thickness description and a percentage, with the girdle finish (faceted, polished or bruted).","related":["girdle-thickness","knife-edge-girdle","inscription","crown","pavilion","bearded-girdle"],"url":"https://digitalgemology.com/learn/glossary/girdle","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness"]},{"term":"Girdle facets","slug":"girdle-facets","aka":["girdle band"],"category":"Anatomy and proportions","definition":"Girdle facets are the small facets cut around the girdle itself, such as the 16 on the T73.","detail":"They are counted apart from the 57 or 58 facets of a round brilliant. A faceted girdle is a recent habit. It gives a setter an edge to hold, but it adds depth and weight. At the same 8.15 mm width, the T73 weighs 1.99 ct against 1.81 ct for the knife-edged T57.","on_report":"The report says whether the girdle is faceted, polished or bruted.","related":["girdle","knife-edge-girdle","facet","round-brilliant"],"url":"https://digitalgemology.com/learn/glossary/girdle-facets","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness","https://digitalgemology.com/learn/historic-cuts/peruzzi-cut-diamond"]},{"term":"Girdle thickness","slug":"girdle-thickness","aka":["girdle description","Thin to Medium","Slightly Thick","Extremely Thin","Extremely Thick"],"category":"Anatomy and proportions","definition":"Girdle thickness is how thick the girdle is at its thinnest and thickest points. A report gives it as a range of words, such as \"Thin to Medium\".","detail":"The eight words are: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick and Extremely Thick. A round's girdle has 16 valleys and 16 hills. The grader reads it at the valleys, in profile at 10x. Too thin is a chipping risk and too thick adds hidden weight; GIA allows Excellent for Thin to Slightly Thick.","on_report":"On the proportion diagram as a range of words plus an average percentage, and the girdle finish. IGI's Ideal band is Thin to Slightly Thick.","related":["girdle","knife-edge-girdle","durability-cut-grade","weight-ratio","total-depth"],"url":"https://digitalgemology.com/learn/glossary/girdle-thickness","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"GLB file","slug":"glb-file","aka":["GLB","glTF"],"category":"Cutting and files","definition":"GLB is glTF, the web 3D format, in a single file. It loads in three.js, Babylon.js and Google's model-viewer, for the web, AR and real-time 3D.","detail":"A .gltf file with a separate .bin file cannot open in a browser on its own. Use the single .glb file instead. Blender and Godot also open GLB.","how_we_measure":"Our 3D files include GLB. Our tools read your own .glb, or a .gltf with its data inside it. They do not read Draco or meshopt compression.","related":["mesh","3d-model"],"url":"https://digitalgemology.com/learn/glossary/glb-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Grain center","slug":"grain-center","category":"Clarity characteristics","definition":"A grain center is a small, concentrated area of crystal distortion inside a diamond. It can be white or dark and can look like a thread or a pinpoint.","detail":"It comes from distortion in the crystal, not from a foreign mineral. GIA counts it as an inclusion.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["internal-graining","pinpoint","inclusion"],"url":"https://digitalgemology.com/learn/glossary/grain-center","sources":[]},{"term":"Green beryl","slug":"green-beryl","category":"Colored gemstones","definition":"Green beryl is a green beryl that does not qualify as emerald. Its color is too weak, or iron rather than chromium or vanadium mainly causes it.","detail":"LMHC defines emerald as beryl colored mainly by chromium and/or vanadium, with medium to strong green saturation. It calls an iron-rich beryl with only low traces of chromium green beryl. LMHC is still preparing its full wording for green beryl.","on_report":"Identified as beryl and named green beryl, not emerald.","related":["beryl","emerald","saturation","tone","gem-variety"],"url":"https://digitalgemology.com/learn/glossary/green-beryl","sources":[]},{"term":"Guide minimum pavilion angle","slug":"guide-minimum-pavilion-angle","aka":["minimum pavilion angle"],"category":"Gemstone properties","definition":"The guide minimum pavilion angle is the shallowest pavilion at which a round still returns head-on light through the table. It is the larger of the critical angle and (180° − critical angle) ÷ 4.","detail":"It is 38.9° for diamond, 39.5° for moissanite and 38.1° for cubic zirconia. It is 36.4° for ruby and sapphire, 39.4° for emerald and 40.4° for amethyst. Faceters usually cut a degree or two steeper, for light that arrives off the axis.","how_we_measure":"Each gemstone page gives the figure for a round design, from the lower end of the stone's refractive index. Fancy shapes vary, so every cut page has its own head-on test.","related":["critical-angle","pavilion-angle","head-on-light","windowing"],"url":"https://digitalgemology.com/learn/glossary/guide-minimum-pavilion-angle","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/gemstones/diamond"]},{"term":"Hardness (Mohs)","slug":"hardness","aka":["Mohs hardness","hardness","scratch resistance"],"category":"Gemstone properties","definition":"Hardness is a gem's resistance to scratching. Gemologists rank it on the Mohs scale, on which diamond is 10.","detail":"Diamond is 10, moissanite 9.25, ruby and sapphire 9, and cubic zirconia 8 to 8.5. Emerald is 7.5 to 8 and amethyst 7. Mohs is a ranking, not a ruler: diamond is far harder than corundum. Hardness is not toughness, so a hard stone can still chip.","how_we_measure":"Our gemstone pages give each stone's Mohs hardness.","related":["mohs-scale","toughness","durability","cleavage","scratch"],"url":"https://digitalgemology.com/learn/glossary/hardness","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care","https://digitalgemology.com/gemstones/diamond","https://digitalgemology.com/gemstones/ruby","https://digitalgemology.com/gemstones/emerald","https://digitalgemology.com/gemstones/amethyst","https://digitalgemology.com/gemstones/cubic-zirconia"]},{"term":"Head","slug":"head","aka":["end","point","tip"],"category":"Anatomy and proportions","definition":"The head, or end, is each end of an elongated stone such as an oval, pear or marquise. On a marquise the ends are points.","detail":"GIA's oval guide uses head or end, shoulder and belly to name the parts of an elongated stone. Pointed ends are weak spots, so settings usually protect them, for example with V-prongs.","related":["shoulder","belly","marquise-cut","pear-cut"],"url":"https://digitalgemology.com/learn/glossary/head","sources":["https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds","https://digitalgemology.com/cuts/marquise"]},{"term":"Head shadow","slug":"head-shadow","aka":["observer shadow"],"category":"How we measure","definition":"The head shadow is the part of the overhead light that the viewer's own head blocks. In our tools, it is a cone around the line of sight in which no light counts.","detail":"A well-cut round sends much of its light back from straight overhead. So the head shadow causes a dip in return near face-up, and it draws the contrast pattern. Each tool uses its own head shadow, which is one reason their figures differ.","how_we_measure":"Simulator: no light within 6°, fading back in by 10.5°. Brightness & Tilt Analysis: 0° to 30°, which you set; the default is 10°. Cut Grading Tool brilliance: 23°, after Hemphill et al. (1998) and Moses et al. (2004). Light census and Cut Grading Tool light maps: none.","related":["contrast","brightness-and-tilt-analysis","iso-lighting","light-return","dome-model"],"url":"https://digitalgemology.com/learn/glossary/head-shadow","sources":["https://digitalgemology.com/about/how-we-measure/light-measures","https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/about/how-we-measure/cut-grading-tool","https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"Head-on light","slug":"head-on-light","aka":["pavilion reflecting head-on light","head-on test"],"category":"Light performance","definition":"Head-on light is light that enters a stone straight down along its axis. Our head-on test gives the share of the pavilion whose facets are steep enough to reflect that light.","detail":"A facet reflects head-on light only if it lies at or above the critical angle from the girdle plane. The test checks the first reflection only; it does not trace where the light goes next.","how_we_measure":"Cut pages give “Pavilion reflecting head-on light” as a share of the pavilion, by projected area. The verdict reads “Holds head-on light” at 99% or more, and “Minor windowing” from 90%. Below 90% it reads “Steepen the pavilion”. A culet flat counts as not reflecting, and a cut with no table is not measured.","related":["critical-angle","windowing","guide-minimum-pavilion-angle","pavilion-angle","total-internal-reflection"],"url":"https://digitalgemology.com/learn/glossary/head-on-light","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"Hearts & Arrows Scope","slug":"hearts-and-arrows-scope","aka":["H&A Scope"],"category":"How we measure","definition":"The Hearts & Arrows Scope is a free Digital Gemology tool that simulates a hearts and arrows viewer. It shows a round brilliant's arrows face-up and hearts face-down, with one symmetry figure per view.","detail":"You can change the pavilion angle, crown angle and table size. You can also twist the crown, tip or turn the pavilion mains, or move the culet off center. Proportion changes alter the pattern's shape; symmetry faults break its match.","how_we_measure":"The scope traces each cut's own 3D model with parallel rays straight down the axis, for up to 24 bounces. It has no Fresnel loss, dispersion or double refraction. The same viewer draws the hearts and arrows images on the gemstone pages.","related":["hearts-and-arrows","hearts-and-arrows-viewer","hearts-and-arrows-symmetry","symmetry"],"url":"https://digitalgemology.com/learn/glossary/hearts-and-arrows-scope","sources":["https://digitalgemology.com/tools/hearts-and-arrows-scope","https://digitalgemology.com/about/how-we-measure/hearts-and-arrows-scope"]},{"term":"Hearts and arrows","slug":"hearts-and-arrows","aka":["H&A","hearts & arrows pattern","hearts & arrows","Heart & Cupid"],"category":"Light performance","definition":"Hearts and arrows is a pattern that a precisely cut round brilliant shows in a reflector viewer. Face-up it shows eight arrows, and face-down eight hearts.","detail":"The pattern appears only when opposite facets match in angle and direction. It shows cutting precision, not how much light a stone returns. It also needs angles that suit the stone's index: the T73 loses the pattern in moissanite. GIA and AGS do not grade hearts and arrows.","how_we_measure":"The Hearts & Arrows Scope traces both views from each cut's own model, with a symmetry figure for each. Five cuts carry the pattern there: the T73, the T105, DG-Vitruvio, DG-Anteros and DG-Broken Hearts and Arrows. In moissanite, the T73's pattern falls apart while its symmetry stays at 100%.","related":["hearts-and-arrows-scope","hearts-and-arrows-viewer","hearts-and-arrows-symmetry","symmetry","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/hearts-and-arrows","sources":["https://digitalgemology.com/about/how-we-measure/hearts-and-arrows-scope","https://digitalgemology.com/learn/historic-cuts/hearts-and-arrows-diamonds","https://digitalgemology.com/tools/hearts-and-arrows-scope"]},{"term":"Hearts and arrows symmetry","slug":"hearts-and-arrows-symmetry","aka":["H&A symmetry figure"],"category":"How we measure","definition":"Hearts and arrows symmetry is the share of a hearts and arrows pattern where all sixteen halves match. Our Hearts & Arrows Scope gives it as a whole percent for each view.","detail":"Proportions alone never lower it, because they change every half in the same way. Small faults do: in the T73 in diamond, mains up to 0.1° off give 90% face-up and 85% face-down. It differs from a report symmetry grade and from our light-performance symmetry score.","how_we_measure":"We lay a polar grid of 32 angles by 96 radii over one half-sector. We trace each point in all 16 of its images. A point counts when all 16 show the same color, weighted by its radius. 100% means every point agrees.","related":["hearts-and-arrows","hearts-and-arrows-scope","symmetry-light-performance","symmetry"],"url":"https://digitalgemology.com/learn/glossary/hearts-and-arrows-symmetry","sources":["https://digitalgemology.com/about/how-we-measure/hearts-and-arrows-scope","https://digitalgemology.com/tools/hearts-and-arrows-scope"]},{"term":"Hearts and arrows viewer","slug":"hearts-and-arrows-viewer","aka":["reflector viewer","H&A viewer"],"category":"How we measure","definition":"A hearts and arrows viewer is a reflector scope that colors a stone by the direction its light comes from. In it, a precisely cut round brilliant shows eight arrows face-up and eight hearts face-down.","detail":"Real viewers differ in their geometry. In Kinsaku Yamashita's patented design, the white tube's rim sits 10° to 25° off the axis. The best patterns show at about 15° to 20°.","how_we_measure":"Our viewer colors each ray by the angle θ between its exit direction and the viewing axis. Below 2.5° is the lens port, drawn dark. From 2.5° to under 13.5° is the white disk, which draws the arrows and hearts. From 13.5° to 90° is the red reflector; light heading away, or still inside after 24 bounces, is dark.","related":["hearts-and-arrows","hearts-and-arrows-scope","hearts-and-arrows-symmetry"],"url":"https://digitalgemology.com/learn/glossary/hearts-and-arrows-viewer","sources":["https://digitalgemology.com/about/how-we-measure/hearts-and-arrows-scope","https://digitalgemology.com/learn/historic-cuts/hearts-and-arrows-diamonds"]},{"term":"Heat treatment","slug":"heat-treatment","aka":["heating","heated"],"category":"Treatments and origin","definition":"Heat treatment is controlled heating of a gem to change its color or clarity.","detail":"It is used on ruby, sapphire and amethyst, among others. GIA considers heated gems durable and permanent in normal wear. Most citrine is heated amethyst. Heating with an added element, such as beryllium, is a separate treatment: diffusion.","on_report":"Corundum: 'indications of heating (H)' or 'no indications of heating (NH)'.","related":["indications-of-heating","no-indications-of-heating","residues-in-healed-fissures","beryllium-diffusion","silk"],"url":"https://digitalgemology.com/learn/glossary/heat-treatment","sources":[]},{"term":"Heavy liquid","slug":"heavy-liquid","aka":["SG liquid"],"category":"Gem testing","definition":"A heavy liquid is a dense liquid that sorts stones by specific gravity. A lighter stone floats in it, and a denser one sinks.","detail":"In methylene iodide (SG 3.32), moissanite (SG 3.22) floats and diamond (SG 3.52) sinks. The liquid is toxic, so this is a laboratory method.","related":["specific-gravity","thermal-diamond-tester"],"url":"https://digitalgemology.com/learn/glossary/heavy-liquid","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-carat-size"]},{"term":"High-order white","slug":"high-order-white","category":"Optics","definition":"High-order white is the near-white light that a thick doubly refractive stone passes between crossed polarizers. Its two rays are so many wavelengths apart that the interference colors mix into white.","detail":"A ruby 5.8 mm thick, across its axis, puts its rays about 46 µm apart. That is some 80 wavelengths of green light. The white takes on the stone's own body color. True interference colors show only near the optic axis or in the conoscope.","how_we_measure":"The Polariscope takes the exact average of each 5 nm band. So it does not invent colors that are not there.","related":["polariscope","birefringence","interference-figure"],"url":"https://digitalgemology.com/learn/glossary/high-order-white","sources":["https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"HPHT","slug":"hpht","aka":["high pressure, high temperature","HPHT-processed","HPHT treatment","HPHT growth"],"category":"Treatments and origin","definition":"HPHT stands for high pressure, high temperature. The same conditions can grow a diamond in a laboratory or treat a natural one to change its color.","detail":"When HPHT is a treatment, it is permanent and must be disclosed. GIA grades an HPHT-treated diamond on a full grading report, never a Dossier.","on_report":"For HPHT color treatment, GIA adds an asterisk to the color grade and explains it in Comments. It inscribes TREATED COLOR on the girdle. For HPHT growth, the stone gets a lab-grown report; IGI can note HPHT as the growth method.","related":["cvd","color-treatment","asterisk-color-grade","laboratory-grown-diamond","treatment"],"url":"https://digitalgemology.com/learn/glossary/hpht","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences"]},{"term":"Hue","slug":"hue","category":"Colored gemstones","definition":"Hue is the basic color of a gem, such as red, blue or green. A second color can modify it, as in purplish red.","detail":"Hue can decide the variety. Red must be the dominant hue before GIA calls a stone ruby. In ruby, pure red to slightly purplish red is valued most.","on_report":"Colored stone reports state the color in words, such as 'purplish red'.","related":["tone","saturation","gem-variety","pink-sapphire","trade-color-term"],"url":"https://digitalgemology.com/learn/glossary/hue","sources":["https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence"]},{"term":"Hue, tone and saturation","slug":"hue-tone-saturation","category":"Color and fluorescence","definition":"Hue, tone and saturation are the three parts of a color. Hue is the color itself, tone is how light or dark it is, and saturation is how strong it is.","detail":"Fancy color grading uses all three. The hue names the color, and one fancy-grade word describes the effect of tone and saturation together.","on_report":"Combined in the fancy color grade, for example \"Fancy Vivid Blue\".","related":["fancy-color-grade","fancy-color-diamond","hue","tone","saturation"],"url":"https://digitalgemology.com/learn/glossary/hue-tone-saturation","sources":["https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence"]},{"term":"Hydrothermal growth","slug":"hydrothermal-growth","aka":["hydrothermal synthetic"],"category":"Treatments and origin","definition":"Hydrothermal growth makes synthetic crystals in a hot water solution under high pressure.","detail":"It is the only successful method for growing synthetic quartz, including synthetic amethyst. Hydrothermal synthetic emerald for jewelry appeared in 1960. These emeralds often show zigzag, chevron-like growth lines.","on_report":"Labs report the stone as laboratory-grown (synthetic).","related":["synthetic-gemstone","flux-growth","quartz","emerald","amethyst"],"url":"https://digitalgemology.com/learn/glossary/hydrothermal-growth","sources":[]},{"term":"Ideal cut","slug":"ideal-cut","aka":["Ideal","Excellent-Ideal","Ideal Cut Round Brilliant","ideal proportions"],"category":"Grading report","definition":"\"Ideal cut\" has three meanings: an IGI comment, a GIA supplement grade, or a sales word. At IGI it marks an Excellent round with narrower proportions; at GIA it appears only as \"AGS Ideal\".","detail":"IGI's May 2023 Ideal band starts with total depth 58.5 to 62.5% and table 53 to 60%. The crown angle must be 33 to 36° and the crown height 13 to 16.5%. The pavilion must be 40.0 to 41.3° and 42 to 44% deep, with a Thin to Slightly Thick girdle. Polish and symmetry must be at least Very Good.","on_report":"IGI writes \"Ideal Cut Round Brilliant\" as a comment beside an Excellent cut grade. GIA prints no Ideal cut grade.","related":["cut-grade","ags-ideal-report","triple-excellent","tolkowsky-ideal-cut"],"url":"https://digitalgemology.com/learn/glossary/ideal-cut","sources":["https://digitalgemology.com/learn/grading-reports/what-ideal-cut-means","https://digitalgemology.com/learn/grading-reports/how-to-read-a-diamond-grading-report"]},{"term":"Identification Report","slug":"identification-report","aka":["ID report","gem identification report"],"category":"Grading report","definition":"An Identification Report states what a material is, without color or clarity grades. GIA issues one for simulants and for diamonds with unstable treatments, such as coating or fracture filling.","detail":"GIA doesn't grade simulants or unstably treated diamonds, so it identifies them instead. Expect no D-to-Z letter and no clarity grade. IGI also issues identification reports, for example for moissanite that it doesn't grade.","on_report":"The report names the material, whether it is natural or laboratory-grown, and any detectable treatment.","related":["simulant","moissanite","fracture-filling","coating","diamond-grading-report"],"url":"https://digitalgemology.com/learn/glossary/identification-report","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Included (I1, I2, I3)","slug":"included","aka":["I1","I2","I3","P1","P2","P3","piqué"],"category":"Four Cs","definition":"I1, I2 and I3 are the lowest clarity grades. Their inclusions are obvious at 10x and can affect transparency and brilliance.","detail":"I3 is the bottom of GIA's scale. In our simulator, an I3 stone plots eight characteristics, against one at VVS1.","on_report":"Written I1, I2 or I3 in the clarity grade line.","related":["si","clarity-grade","cloud","feather"],"url":"https://digitalgemology.com/learn/glossary/included","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Inclusion","slug":"inclusion","aka":["inclusions","internal characteristic"],"category":"Clarity characteristics","definition":"An inclusion is a clarity characteristic inside a diamond, or one that reaches the surface from inside.","detail":"Small crystals can be trapped while a diamond grows, and the crystal itself can grow irregularly. Crystals, pinpoints, clouds, needles and feathers are inclusions.","on_report":"Named in the key to symbols. On GIA's plot most inclusions are red. Cavities, indented naturals, knots, etch channels and laser drill holes are red and green.","related":["blemish","crystal-inclusion","pinpoint","feather","cloud","clarity-grade"],"url":"https://digitalgemology.com/learn/glossary/inclusion","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Indented natural","slug":"indented-natural","category":"Clarity characteristics","definition":"An indented natural is a part of the rough diamond's original surface that dips below the polished surface.","detail":"A natural stays at or above the polished surface, so GIA counts it as a blemish. An indented natural reaches into the stone, so GIA counts it as an inclusion.","on_report":"Drawn in red and green together on the clarity plot of a full GIA report. A Dossier has no plot but can name it in its list of up to four.","related":["natural","girdle","inclusion"],"url":"https://digitalgemology.com/learn/glossary/indented-natural","sources":[]},{"term":"Index gear","slug":"index-gear","aka":["index wheel","index"],"category":"Cutting and files","definition":"An index gear is the toothed wheel on a faceting machine that sets each facet's direction around the stone. Each index is one tooth of the gear.","detail":"Most of our designs use the common 96-tooth gear; a few use 64 or 120. On a 96 gear, the T73's pavilion mains sit every 12 teeth.","how_we_measure":"Each design on our cut pages names its index gear. Our cutting-file check turns each facet's angle and index into a direction and compares it with the 3D model.","related":["facet","tier","faceting-machine","cutting-file-accuracy"],"url":"https://digitalgemology.com/learn/glossary/index-gear","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/3d-models"]},{"term":"Indications of heating","slug":"indications-of-heating","aka":["H"],"category":"Treatments and origin","definition":"'Indications of heating' (H) is the standard LMHC lab wording for a corundum that shows signs of heat treatment. The heating changed its color or transparency.","detail":"Heating can leave some inclusions, such as intact rutile needles, unchanged while it alters others. If the lab sees heat-altered inclusions or spectra that show heating, it may write 'indications of (low temperature) heating'. Residues in healed fissures get their own rating.","on_report":"Further information: indications of heating (H), with any residues in healed fissures rated.","related":["no-indications-of-heating","heat-treatment","residues-in-healed-fissures","silk"],"url":"https://digitalgemology.com/learn/glossary/indications-of-heating","sources":[]},{"term":"Indirect light (Light Return)","slug":"indirect-light","aka":["indirect","indirect light (Light Return)"],"category":"Light performance","definition":"Indirect light is light that reaches the viewer from below 45° above the girdle plane. Our light map shows it in green.","detail":"Low-angle light is softer and less dependable than light from above. In moissanite, the T73 trades much of its direct light for indirect light.","how_we_measure":"The simulator's light map counts light from below 45° elevation as indirect.","related":["light-return","light-map","direct-light","contrast"],"url":"https://digitalgemology.com/learn/glossary/indirect-light","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-refractive-index-and-cut"]},{"term":"Inscription","slug":"inscription","aka":["laser inscription","girdle inscription","Laserscribe","inscription(s)"],"category":"Grading report","definition":"An inscription is text that a laser writes on the girdle of a stone, too small to see without magnification. Most often it is the report number.","detail":"It ties the stone to its report. GIA inscribes the number on every Dossier stone, and TREATED COLOR on an HPHT color-treated stone. GIA inscribes lab-grown stones with \"Laboratory-Grown\" and the report or assessment number. Check the lettering as well as the number: a fake inscription can copy a real number.","on_report":"The report lists any inscription under Inscription(s). IGI calls its inscription Laserscribe. It inscribes every lab-grown stone with \"LG\" before the number, and natural stones on request.","related":["report-number","girdle","diamond-dossier","laboratory-grown-diamond","report-check"],"url":"https://digitalgemology.com/learn/glossary/inscription","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Interference figure","slug":"interference-figure","aka":["conoscopic figure","optic figure"],"category":"Optics","definition":"An interference figure is the pattern of colors and dark bands that a doubly refractive stone shows through a conoscope. Each point of the figure is one direction through the stone.","detail":"Its center, the melatope, marks the optic axis. A uniaxial stone seen down its axis shows a dark cross centered on the melatope, often with colored rings. Quartz, such as amethyst, shows a bull's-eye with no cross at the center, because of its optical activity. A biaxial stone shows a different figure.","how_we_measure":"The Polariscope computes the figure by Jones calculus over a 40° cone of directions. The melatope leaves the view when the stone tilts past 14° (moissanite) to 25° (amethyst).","related":["conoscope","optic-axis","optic-sign","red-plate","optical-activity"],"url":"https://digitalgemology.com/learn/glossary/interference-figure","sources":["https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Internal graining","slug":"internal-graining","aka":["graining","growth lines"],"category":"Clarity characteristics","definition":"Internal graining is lines, angles or curves inside a diamond caused by irregular crystal growth. At 10x they can look whitish, colored or reflective, or lower the transparency.","detail":"It comes from irregular crystal growth, not from a foreign crystal. GIA's 1997 moissanite study lists graining among the features that a diamond can show.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["surface-graining","grain-center","twinning-wisp","inclusion"],"url":"https://digitalgemology.com/learn/glossary/internal-graining","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Internal laser drilling","slug":"internal-laser-drilling","aka":["ILD"],"category":"Clarity characteristics","definition":"Internal laser drilling is laser work inside a diamond that creates or widens a surface-reaching feather around a dark inclusion. The feather lets a bleach in to make the inclusion less visible.","detail":"It leaves a feather, not a straight drill hole, so it can look like a natural break. It is a clarity treatment, so the report must disclose it.","on_report":"Disclosed in Comments as a treatment.","related":["laser-drilling","laser-drill-hole","feather","treatment"],"url":"https://digitalgemology.com/learn/glossary/internal-laser-drilling","sources":[]},{"term":"Internally Flawless (IF)","slug":"internally-flawless","aka":["IF"],"category":"Four Cs","definition":"Internally Flawless (IF) is the clarity grade for a diamond with no inclusions at 10x. Only blemishes on the surface show.","detail":"It sits one step below Flawless on GIA's 11-grade scale. The difference is only on the surface, such as small polish marks.","on_report":"Written \"Internally Flawless\" or \"IF\".","related":["flawless","vvs","blemish","clarity-grade"],"url":"https://digitalgemology.com/learn/glossary/internally-flawless","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Irradiation","slug":"irradiation","aka":["irradiated"],"category":"Treatments and origin","definition":"Irradiation is exposing a gem to artificial radiation to change its color. Heating sometimes follows, to modify the color further.","detail":"Some irradiated colors fade in strong light. Irradiated yellow sapphire, for example, fades fast when held near a flame. An irradiated stone cannot be called padparadscha.","on_report":"Labs state the treatment when they detect it.","related":["treatment","stability","padparadscha","disclosure"],"url":"https://digitalgemology.com/learn/glossary/irradiation","sources":[]},{"term":"ISO lighting","slug":"iso-lighting","aka":["ISO dome","isotropic dome"],"category":"How we measure","definition":"ISO is a dome model that lights the stone evenly from every direction above the horizon. It asks only how much light comes back, not from where.","detail":"It is the most forgiving of the three dome models. The definition follows Robert W. Strickland's GemRay user guide; the implementation is our own.","how_we_measure":"In the Brightness & Tilt Analysis, ISO gives every direction above the horizon the weight W = 1. The weight is 0 inside the head shadow. The Cut Grading Tool's brilliance is the ISO figure with a 23° head shadow, averaged over 13 tilts.","related":["dome-model","cos-lighting","sc2-lighting","brightness-and-tilt-analysis","head-shadow","cut-grading-tool"],"url":"https://digitalgemology.com/learn/glossary/iso-lighting","sources":["https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/tools/brightness-tilt-analysis","https://digitalgemology.com/about/how-we-measure/cut-grading-tool"]},{"term":"Jardin","slug":"jardin","aka":["garden"],"category":"Clarity characteristics","definition":"Jardin, French for garden, is a trade word for the mossy, garden-like inclusions typical of emerald.","detail":"Eye-clean emeralds are rare, so buyers expect some jardin. Inclusions lower value most when they threaten durability or reduce transparency. Many fractures that reach the surface are filled with oil or resin.","related":["emerald","fissure","three-phase-inclusion","clarity-enhancement","inclusion"],"url":"https://digitalgemology.com/learn/glossary/jardin","sources":[]},{"term":"Jones calculus","slug":"jones-calculus","category":"Optics","definition":"Jones calculus is a matrix method for following polarized light through filters, retarders and crystals. R.C. Jones published it in 1941.","detail":"Each element is a 2 × 2 matrix acting on the light's vibration. Between crossed polarizers, a plain retarder passes I = sin²2φ sin²(γ/2) of the light. Here φ is the turn of the retarder and γ is its retardance.","how_we_measure":"The Polariscope computes everything by Jones calculus at 81 wavelengths. It uses the refractive indices and measured absorption of each stone.","related":["polariscope","polarized-light","birefringence"],"url":"https://digitalgemology.com/learn/glossary/jones-calculus","sources":["https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Kashmir sapphire","slug":"kashmir-sapphire","aka":["Kashmir blue"],"category":"Colored gemstones","definition":"A Kashmir sapphire is a blue sapphire from the Kashmir region on the India/Pakistan border. 'Kashmir blue' is a trade name for a deep, velvety blue, with no single agreed standard.","detail":"Many fine Kashmir sapphires contain tiny inclusions that give a velvety look without loss of transparency. 'Kashmir' is a claim of origin, so only a lab origin report can support it. A 'Kashmir-like' color does not prove it.","on_report":"An origin report can state the opinion 'Kashmir'. Some labs also add color terms, each under its own criteria.","related":["sapphire","origin-report","royal-blue","trade-color-term","silk"],"url":"https://digitalgemology.com/learn/glossary/kashmir-sapphire","sources":[]},{"term":"Keel","slug":"keel","aka":["keel line"],"category":"Anatomy and proportions","definition":"The keel is the short line at the bottom of the pavilion of an elongated stone, such as a marquise. A round has a point there.","detail":"All pavilion facets slope down from the girdle to the keel. Facets that start near the center must slope steeply, and facets far out toward a point slope gently. That spread of angles is why elongated stones can show a bow-tie.","related":["belly","culet","bow-tie","marquise-cut"],"url":"https://digitalgemology.com/learn/glossary/keel","sources":["https://digitalgemology.com/learn/bow-tie/which-diamond-shapes-have-a-bow-tie"]},{"term":"Key to symbols","slug":"key-to-symbols","aka":["key","symbol key"],"category":"Grading report","definition":"The key to symbols is the list beside the clarity plot. It names the kinds of clarity characteristic on that plot.","detail":"GIA lists them by importance to the grade. A laser drill hole comes first, then grade-setting inclusions, other inclusions, naturals and extra facets. On the plot, red shows most inclusions, green shows naturals and black shows extra facets. Cavities, indented naturals, knots, etch channels and laser drill holes use red and green together.","on_report":"Beside the plotted diagram on a full GIA Diamond Grading Report.","related":["clarity-plot","laser-drill-hole","inclusion","blemish","natural","extra-facet"],"url":"https://digitalgemology.com/learn/glossary/key-to-symbols","sources":["https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Knife-edge girdle","slug":"knife-edge-girdle","aka":["knife edge"],"category":"Anatomy and proportions","definition":"A knife-edge girdle has no girdle band at all: the crown and the pavilion meet in a sharp line.","detail":"Tolkowsky's 1919 plan, our T57, has one. It saves weight but chips easily.","on_report":"Described as Extremely Thin on the proportion diagram.","related":["girdle","girdle-thickness","tolkowsky-ideal-cut","girdle-facets"],"url":"https://digitalgemology.com/learn/glossary/knife-edge-girdle","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness","https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/historic-cuts/tolkowsky-ideal-cut-1919"]},{"term":"Knot","slug":"knot","aka":["knots"],"category":"Clarity characteristics","definition":"A knot is a white or transparent diamond crystal that reaches the surface after the stone is cut.","detail":"A knot is a diamond crystal inside the diamond that reaches a polished facet. GIA lists it among the inclusions.","on_report":"Drawn in red and green together on the clarity plot of a full GIA report. A Dossier has no plot but can name it in its list of up to four.","related":["crystal-inclusion","cavity","inclusion"],"url":"https://digitalgemology.com/learn/glossary/knot","sources":[]},{"term":"Laboratory-grown diamond","slug":"laboratory-grown-diamond","aka":["lab-grown diamond","lab-created diamond","synthetic diamond","LGD","LG"],"category":"Treatments and origin","definition":"A laboratory-grown diamond is a diamond that people grow with machines, by HPHT or CVD. It has essentially the same composition, crystal structure and properties as a natural diamond.","detail":"It is diamond, not a simulant. In our simulator it renders identically to a natural diamond, pixel for pixel; only a lab can tell them apart. Do not confuse \"lab-grown diamond\" with \"lab-grown moissanite\": both are made in a lab, but only one is diamond.","on_report":"Since 1 October 2025, GIA rates a colorless to near-colorless lab-grown diamond Premium or Standard, with no D-to-Z or clarity grade. It inscribes \"Laboratory-Grown\" and the assessment number. IGI inscribes \"LG\" before the report number and can note the growth method in Comments.","related":["natural-diamond","hpht","cvd","laboratory-grown-diamond-report","simulant"],"url":"https://digitalgemology.com/learn/glossary/laboratory-grown-diamond","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Laboratory-Grown Diamond Quality Assessment","slug":"laboratory-grown-diamond-quality-assessment","aka":["GIA lab-grown diamond assessment","Premium or Standard"],"category":"Grading report","definition":"GIA's Laboratory-Grown Diamond Quality Assessment rates a lab-grown diamond Premium or Standard. It replaced GIA's D-to-Z lab-grown reports on 1 October 2025.","detail":"It covers loose, colorless to near-colorless lab-grown diamonds of 0.15 ct or more. GIA gave its reason: most lab-grown diamonds fall in a very narrow range of color and clarity. Premium needs D color, VVS clarity or higher, Excellent polish and symmetry, and Excellent cut for a round. A stone below the Standard minimums gets no assessment.","on_report":"The assessment states Premium or Standard, not a color or clarity grade. GIA inscribes \"Laboratory-Grown\" and the assessment number on the girdle.","related":["laboratory-grown-diamond","laboratory-grown-diamond-report","diamond-grading-report","inscription"],"url":"https://digitalgemology.com/learn/glossary/laboratory-grown-diamond-quality-assessment","sources":[]},{"term":"Laboratory-Grown Diamond Report","slug":"laboratory-grown-diamond-report","aka":["GIA Laboratory-Grown Diamond Report","lab-grown diamond report","synthetic diamond grading report","LGDR"],"category":"Grading report","definition":"The Laboratory-Grown Diamond Report was GIA's digital-only report for laboratory-grown diamonds, from October 2020 to September 2025. It gave color and clarity grades and stated the growth method, CVD or HPHT.","detail":"GIA designed it to look distinctly different from its reports for natural diamonds. GIA inscribed each stone with the report number and the word LABORATORY-GROWN. Since 1 October 2025, GIA issues a Laboratory-Grown Diamond Quality Assessment instead. It calls a stone Premium or Standard and gives no D-to-Z or clarity grades.","on_report":"The report type itself states the origin. IGI marks lab-grown stones with \"LG\" before the report number, and Comments can note the growth method on request.","related":["laboratory-grown-diamond","natural-diamond","inscription","hpht","cvd","diamond-grading-report","laboratory-grown-diamond-quality-assessment"],"url":"https://digitalgemology.com/learn/glossary/laboratory-grown-diamond-report","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Laser drill hole","slug":"laser-drill-hole","aka":["LDH","laser drill-hole"],"category":"Clarity characteristics","definition":"A laser drill hole is a tiny, surface-reaching tunnel made by a laser beam, usually toward a dark inclusion.","detail":"It is the trace of a laser drilling treatment. GIA grades a laser-drilled stone and discloses the treatment.","on_report":"Listed first in GIA's key to symbols and drawn in red and green together; Comments describes the treatment.","related":["laser-drilling","internal-laser-drilling","key-to-symbols","comments"],"url":"https://digitalgemology.com/learn/glossary/laser-drill-hole","sources":["https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin"]},{"term":"Laser drilling","slug":"laser-drilling","aka":["laser-drilled","LDH treatment"],"category":"Treatments and origin","definition":"Laser drilling is a clarity treatment that bores a tiny channel to an inclusion, usually a dark one. The laser then burns the inclusion away, or a bleaching agent enters through the channel.","detail":"It is permanent, so GIA grades a laser-drilled stone and discloses the treatment. IGI also discloses permanent treatments in Comments.","on_report":"The drill hole appears in the key to symbols, and Comments describes the treatment.","related":["laser-drill-hole","internal-laser-drilling","treatment","key-to-symbols"],"url":"https://digitalgemology.com/learn/glossary/laser-drilling","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Lead-glass filling","slug":"lead-glass-filling","aka":["glass-filled ruby","lead glass-filled ruby"],"category":"Treatments and origin","definition":"Lead-glass filling is a treatment that fills fissures and cavities in corundum, mostly ruby, with high-lead glass to hide them.","detail":"It usually applies to low-quality stones. Colored flashes in the fissures can give it away. The glass is unstable to heat and to some chemicals; even lemon juice can change it. Clean these stones with a damp cloth only.","on_report":"Indications of heating, glass-filled fissures (and cavities), extent minor (F1), moderate (F2) or significant (F3). Badly fractured rough held together by glass is a manufactured product.","related":["manufactured-product","clarity-enhancement","ruby","stability","disclosure"],"url":"https://digitalgemology.com/learn/glossary/lead-glass-filling","sources":[]},{"term":"Leakage","slug":"leakage","aka":["light leakage"],"category":"Light performance","definition":"Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.","detail":"Leakage rises when pavilion facets are too shallow for the stone's critical angle. So stones with a refractive index below 2 usually leak more than diamond in the same cut. Each of our tools counts leakage in its own way. Compare leakage figures only within one tool and one gemstone.","how_we_measure":"Light census (Cut Performance Wheel, cut pages and Index): the share of entering rays that leave through the pavilion. The Wheel's method page also counts rays that leave through the crown heading down. Brightness & Tilt Analysis: rays that leave below the horizon, find no exit, or stay inside after 12 bounces. That figure does not change with the head shadow. Simulator light map: light out through the pavilion, from below the horizon, or still inside after 18 bounces.","related":["light-return","light-map","critical-angle","windowing","overall-score","census-ray-classes"],"url":"https://digitalgemology.com/learn/glossary/leakage","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"Length-to-width ratio","slug":"length-to-width-ratio","aka":["L/W ratio","ratio","length to width"],"category":"Anatomy and proportions","definition":"The length-to-width ratio of a fancy shape is its length divided by its width, both taken from the measurements.","detail":"GIA's oval guide says that many people prefer ovals of 1.30 to 1.50. The ratio can't predict a bow-tie. Our DG-Bowtie and Marquise are both 2.06 to 1, and only DG-Bowtie has a band.","on_report":"Not printed as a line: divide the length by the width from the measurements.","related":["measurements","fancy-shape","bow-tie","oval-cut","marquise-cut"],"url":"https://digitalgemology.com/learn/glossary/length-to-width-ratio","sources":["https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report"]},{"term":"Light (S–Z)","slug":"light-color","aka":["S-Z","Z color"],"category":"Color and fluorescence","definition":"Light is the D-to-Z range from S to Z: a plainly visible yellow or brown tint.","detail":"A Z is not a bad diamond, only one with a visible tint. A yellow or brown diamond with more color than the Z master stone leaves the scale. Then it can get a fancy color grade.","related":["very-light-color","fancy-color-diamond","color-grade"],"url":"https://digitalgemology.com/learn/glossary/light-color","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z","https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence"]},{"term":"Light census","slug":"light-census","aka":["census","face-up light census"],"category":"How we measure","definition":"The light census is our ray tracer behind the Cut Performance Wheel, the cut-page scores and the Cut Performance Index. It drops a fixed set of rays on a stone from above and counts where each one leaves.","detail":"The census is deterministic: the rays come from a fixed seed, so the same cut always gets the same score. It leaves out Fresnel loss, absorption, color, inclusions and polish. So a dark colored stone looks darker than its score.","how_we_measure":"We scale each cut to unit size in plan. For each gemstone, 3,200 rays fall within 0.12 rad (6.9°) of vertical, cosine-weighted, over a disc larger than the stone. Rays that miss do not count. Inside, a ray reflects totally or leaves through the first facet that lets it out, for up to 12 bounces. A second run tilts the stone 20°, with 2,200 rays.","related":["census-ray-classes","ray-tracing","fire-spread","fixed-settings","cut-performance-index"],"url":"https://digitalgemology.com/learn/glossary/light-census","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"Light map","slug":"light-map","aka":["Light Return map"],"category":"How we measure","definition":"A light map is a face-up picture of a cut stone, colored by the direction each point's light came from. Red is direct light, green is indirect light, blue is contrast, and a dark color is leakage.","detail":"Red is light from 45° to 75° above the girdle plane, and green is light from below 45°. Blue is light from 75° and up, within 15° of the line of sight, where the viewer's head is. Leakage is light that leaves through the back of the stone, comes from below the horizon, or stays inside. The simulator draws leakage in black; the Light Return images on our gemstone cut pages draw it in gray.","how_we_measure":"We trace the map face-up and orthographic, with no Fresnel loss, no dispersion and no absorption. So it shows the geometry alone, with no lighting scene. The simulator shows it with its Light map option. The Brightness & Tilt Analysis shows it in its Light Return window. The Cut Grading Tool shows 815 light maps traced on the T73, with no head shadow.","related":["light-return","direct-light","indirect-light","contrast","leakage","aset-map"],"url":"https://digitalgemology.com/learn/glossary/light-map","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds","https://digitalgemology.com/about/how-we-measure/cut-grading-tool","https://digitalgemology.com/gemstones/diamond/old-single"]},{"term":"Light performance","slug":"light-performance","aka":["optical performance"],"category":"Light performance","definition":"Light performance is how well a cut stone handles light. We describe it with brilliance (light returned), fire (color) and scintillation (sparkle).","detail":"It depends on the cut's geometry and on the stone's refractive index and dispersion. A simulation of light performance describes a design, not a real stone. It is not a grading report.","how_we_measure":"We measure it four ways: the light census, Brightness & Tilt Analysis, Cut Grading Tool and simulator. The census is behind the Cut Performance Wheel and the Index. The four answer different questions, so their numbers are not interchangeable.","related":["brilliance","fire","scintillation","light-census","light-return","overall-score"],"url":"https://digitalgemology.com/learn/glossary/light-performance","sources":["https://digitalgemology.com/about/how-we-measure/light-measures","https://digitalgemology.com/about/how-we-measure"]},{"term":"Light Performance Report (IGI)","slug":"igi-light-performance-report","aka":["IGI Light Performance Report"],"category":"Grading report","definition":"IGI's Light Performance Report is an extra report for round diamonds with top optical performance. It gives a light performance score with sub-scores and an Ideal-Scope image made by ray tracing.","detail":"It uses only the stone's external proportions, so it doesn't account for heavy inclusions. Our brilliance, fire and scintillation scores are a different measure, from our own models.","on_report":"A separate IGI report, issued with the grading report.","related":["ags-ideal-report","ideal-cut","brilliance","fire","cut-grade"],"url":"https://digitalgemology.com/learn/glossary/igi-light-performance-report","sources":["https://digitalgemology.com/learn/grading-reports/what-ideal-cut-means"]},{"term":"Light Return","slug":"light-return","aka":["Light Return shares"],"category":"Light performance","definition":"Light Return sorts each point of a face-up stone by the direction its light came from. Its four classes, direct, indirect, contrast and leakage, together cover the whole face-up stone.","detail":"The shares depend on geometry alone: there is no Fresnel loss, no dispersion and no absorption. The light map draws the four classes in color. Industry maps such as the ASET map work on the same idea.","how_we_measure":"The simulator traces the map face-up and orthographic. It classes each ray by the elevation its light came from, above the girdle plane. Contrast is 75° and up, direct 45° to 75°, and indirect below 45°. The simulator's shares come from a 160 × 160 render with one ray per pixel, rounded to whole percent. The Brightness & Tilt Analysis and the Cut Grading Tool use the same zones.","related":["light-map","direct-light","indirect-light","contrast","leakage","aset-map"],"url":"https://digitalgemology.com/learn/glossary/light-return","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/about/how-we-measure/cut-grading-tool","https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds"]},{"term":"Lizard skin","slug":"lizard-skin","category":"Clarity characteristics","definition":"Lizard skin is a wavy or bumpy area on the surface of a polished diamond.","detail":"It is a blemish, so it lies on the surface only. GIA's polish grade covers surface condition, so lizard skin can count there.","on_report":"A blemish; it can count in the polish grade.","related":["polish-lines","polish","blemish"],"url":"https://digitalgemology.com/learn/glossary/lizard-skin","sources":[]},{"term":"Lower half facet","slug":"lower-half-facet","aka":["lower halves","lower girdle facets","lower girdle halves"],"category":"Anatomy and proportions","definition":"Lower half facets are the 16 long, thin triangles along the pavilion edge of a round brilliant. There are two between each pair of pavilion mains.","detail":"Their reach toward the culet is the lower half length, and longer lower halves leave narrower mains. In the decades after 1919, cutters made them longer. That change later shaped the hearts in hearts and arrows stones.","on_report":"Described on the proportion diagram by the lower half length.","related":["lower-half-length","pavilion-main","hearts-and-arrows","pavilion"],"url":"https://digitalgemology.com/learn/glossary/lower-half-facet","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length","https://digitalgemology.com/learn/historic-cuts/hearts-and-arrows-diamonds"]},{"term":"Lower half length","slug":"lower-half-length","aka":["lower girdle facet length","LGF %","lower half %"],"category":"Anatomy and proportions","definition":"Lower half length is how far the lower half facets reach from the girdle toward the culet. It is a percentage of that distance.","detail":"GIA's Excellent range is 70 to 85% and its Very Good range 65 to 90%. By a common rule of thumb, about 75% or less gives fatter hearts in a hearts and arrows viewer. At 80% or more, the hearts are thinner and further apart. The T73's lower halves are at 77%.","on_report":"On the proportion diagram; GIA reports it to the nearest 5%.","related":["lower-half-facet","star-length","hearts-and-arrows","pavilion-main"],"url":"https://digitalgemology.com/learn/glossary/lower-half-length","sources":["https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/historic-cuts/hearts-and-arrows-diamonds"]},{"term":"Luster","slug":"luster","aka":["lustre","adamantine","vitreous"],"category":"Gemstone properties","definition":"Luster is the way the surface of a stone reflects light. It depends mostly on the refractive index and the quality of the polish.","detail":"Diamond has adamantine luster. Moissanite's is subadamantine. Most colored stones, such as emerald, ruby and spinel, have vitreous (glass-like) luster.","related":["refractive-index","fresnel-reflection","polish"],"url":"https://digitalgemology.com/learn/glossary/luster","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties"]},{"term":"Magic sizes","slug":"magic-sizes","aka":["magic weights","round-number weights"],"category":"Four Cs","definition":"Magic sizes are round-number weights, such as half a carat or one carat. They sell for more because of demand, not looks.","detail":"In our simulator, a 0.99 ct T73 is 0.02 mm narrower than a 1.00 ct. Nobody can see that, but the price below the line is usually much lower. GIA's cut booklet notes that cutters can paint or dig out the half facets to save weight. One reason is to reach one carat.","related":["carat-weight","painting-and-digging","weight-ratio"],"url":"https://digitalgemology.com/learn/glossary/magic-sizes","sources":["https://digitalgemology.com/learn/four-cs/carat-weight-vs-size","https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length"]},{"term":"Manufactured product","slug":"manufactured-product","aka":["composite ruby","ruby with glass","corundum with glass","emerald with resin"],"category":"Treatments and origin","definition":"A manufactured product is a gem held together by glass or resin. It is made from badly fractured rough, or from many small pieces bound into one.","detail":"LMHC wording names it, for example, 'ruby with glass' or 'emerald with resin'. If the glass or resin is altered, the stone may fall into pieces. Heat and chemicals can damage it, so remove it from the setting before jewelry repair.","on_report":"Identification: manufactured product, or corundum with/and glass. Further information: this item is a combination of glass and ruby.","related":["lead-glass-filling","resin-filling","clarity-enhancement","disclosure","ruby"],"url":"https://digitalgemology.com/learn/glossary/manufactured-product","sources":[]},{"term":"Marquise cut","slug":"marquise-cut","aka":["marquise","navette"],"category":"Cuts and shapes","definition":"A marquise cut is an elongated brilliant pointed at both ends.","detail":"It is the most stretched common outline, with two points, so it is the hardest to cut without a bow-tie. GIA's marquise guide says that marquises typically show some bow-tie. But our Marquise and DG-Bowtie share a 2.06-to-1 outline and 87 facets, and only DG-Bowtie has a band. The points need protection in a setting, usually V-prongs.","on_report":"No GIA cut grade; measurements as length × width × depth.","related":["fancy-shape","bow-tie","head","keel","pear-cut"],"url":"https://digitalgemology.com/learn/glossary/marquise-cut","sources":["https://digitalgemology.com/cuts/marquise","https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds","https://digitalgemology.com/learn/bow-tie/which-diamond-shapes-have-a-bow-tie","https://digitalgemology.com/learn/bow-tie/how-to-fix-a-bow-tie"]},{"term":"Master stones","slug":"master-stones","aka":["master diamonds","master color comparison diamonds","master set"],"category":"Color and fluorescence","definition":"Master stones are reference diamonds of known color grade. A grader sets a diamond beside them to decide its D-to-Z letter.","detail":"Light and background change how a diamond looks, so labs don't grade color by eye in a showroom. GIA uses master diamonds in a standard viewing environment, with at least two independent graders. IGI also grades against master stones, with several gemologists.","on_report":"Behind every D-to-Z color grade on GIA and IGI reports.","related":["color-grade","colorless","near-colorless"],"url":"https://digitalgemology.com/learn/glossary/master-stones","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z","https://digitalgemology.com/learn/grading-reports/diamond-report-grades"]},{"term":"Mazarin cut","slug":"mazarin-cut","aka":["Mazarin","double cut","double-cut brilliant"],"category":"Cuts and shapes","definition":"The Mazarin, or double cut, is the first brilliant, from the mid-1600s. It has 34 facets in all, on a cushion outline with a wide, open culet.","detail":"It has 16 crown facets plus the table. By tradition it is named after Cardinal Mazarin of France. It is the step between the single cut and the Peruzzi. Face up, its open culet shows as a dark spot.","related":["old-single-cut","peruzzi-cut","culet","brilliant-cut"],"url":"https://digitalgemology.com/learn/glossary/mazarin-cut","sources":["https://digitalgemology.com/learn/historic-cuts/mazarin-cut-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-culet","https://digitalgemology.com/cuts/mazarin"]},{"term":"Measure your own stone","slug":"measure-your-own-stone","aka":["your own file"],"category":"How we measure","definition":"“Measure your own stone” lets three of our tools measure your own cutting file or 3D model. They use the same code and settings as for our cuts.","detail":"The tools are the 3D Gemstone Simulator, the Brightness & Tilt Analysis and the Cut Performance Wheel. The file must describe one closed, convex, faceted stone, with at most 400 facets and up to 32 MB. The tools refuse anything else with a message, rather than measure it wrong.","how_we_measure":"Your browser reads the file on your device and never uploads it. The tools read GemCad (.asc, .gem), Gem Cut Studio (.gcs), STL, OBJ, PLY, OFF, glTF, 3MF, STEP and FBX.","related":["convex","watertight","gemcad-asc","step-file","cut-performance-wheel"],"url":"https://digitalgemology.com/learn/glossary/measure-your-own-stone","sources":["https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Measurements","slug":"measurements","aka":["dimensions","mm measurements"],"category":"Grading report","definition":"Measurements is the report line that gives a stone's size in millimeters. A round reads minimum – maximum diameter × depth, and a fancy shape length × width × depth.","detail":"One cut in one material has only one size at a given weight, so weight and measurements check each other. In our simulator, a 1.00 ct T73 is 6.48 mm across in diamond and 6.67 mm in moissanite. Divide the depth by the average diameter to get the total depth percentage. A big gap between the two diameters shows a round that is out of round.","on_report":"In the identity block of every GIA and IGI diamond report.","related":["average-diameter","total-depth","carat-weight","length-to-width-ratio","moissanite"],"url":"https://digitalgemology.com/learn/glossary/measurements","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section","https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-carat-size"]},{"term":"Meetpoint","slug":"meetpoint","aka":["meet point"],"category":"Cutting and files","definition":"A meetpoint is a point where the corners of neighboring facets meet. Faceters cut each tier to its meetpoints so that the facet pattern closes exactly.","detail":"Sharp, clean meetpoints show that the angles and the cutting order fit together. Faceters check the meetpoints of each tier as they cut.","related":["tier","cutting-order","facet"],"url":"https://digitalgemology.com/learn/glossary/meetpoint","sources":["https://digitalgemology.com/about/files"]},{"term":"Mesh","slug":"mesh","aka":["triangle mesh","polygon mesh"],"category":"Cutting and files","definition":"A mesh describes a 3D surface as many small flat triangles joined at their edges. STL, OBJ, FBX and GLB files store a mesh.","detail":"A mesh of a cut stone needs only a few triangles per facet: the T73 has 118. In a closed mesh, exactly two triangles share every edge.","how_we_measure":"Each build measures every model's volume, size and depth from its mesh. The build stops if a mesh is not closed.","related":["3d-model","watertight","step-file","planar-facet"],"url":"https://digitalgemology.com/learn/glossary/mesh","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/about/files"]},{"term":"Mirror-image symmetry (design)","slug":"mirror-image-symmetry","aka":["8-fold symmetry","design symmetry"],"category":"Cutting and files","definition":"Mirror-image symmetry in a faceting design is the number of identical mirrored sections around the stone. An 8-fold mirror-image design repeats the same pair of mirrored halves eight times.","detail":"It describes the design, not how precisely a real stone is cut. A round brilliant such as the T73 has 8-fold mirror-image symmetry.","how_we_measure":"Our cut pages give each design's symmetry. The Hearts & Arrows Scope averages each facet over its 16 mirror and turn copies. So a cut as designed reads 100%.","related":["symmetry","hearts-and-arrows-symmetry","symmetry-light-performance","index-gear"],"url":"https://digitalgemology.com/learn/glossary/mirror-image-symmetry","sources":["https://digitalgemology.com/cuts/t73-round-brilliant","https://digitalgemology.com/about/how-we-measure/hearts-and-arrows-scope"]},{"term":"Mohs scale","slug":"mohs-scale","aka":["Mohs scale of hardness"],"category":"Gemstone properties","definition":"The Mohs scale ranks minerals from 1 to 10 by which can scratch which. Diamond is 10 and corundum is 9.","detail":"It is a ranking, not a ruler. The steps are not equal: diamond, at 10, is far harder than corundum, at 9. So a one-point difference does not mean the same gap everywhere on the scale.","related":["hardness","durability","toughness"],"url":"https://digitalgemology.com/learn/glossary/mohs-scale","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care"]},{"term":"Moissanite","slug":"moissanite","aka":["silicon carbide","SiC","lab-grown moissanite","synthetic moissanite"],"category":"Diamond and simulants","definition":"Moissanite is silicon carbide, one silicon atom for every carbon atom, in a hexagonal crystal. It is a diamond simulant, and every moissanite in jewelry is laboratory-grown.","detail":"Its refractive index is 2.648 to 2.691, its dispersion 0.104, its specific gravity 3.22 and its hardness Mohs 9.25. It is doubly refractive (birefringence 0.043), so facet edges seen through it can double. Natural moissanite occurs only as tiny crystals; GIA reported the largest known as 4.1 mm. A thermal tester reads moissanite as diamond.","on_report":"GIA issues only an Identification Report for moissanite. Since May 2024, IGI has graded one producer's moissanite. Other moissanite gets an identification report, a certificate of authenticity or a seller's card.","related":["simulant","facet-doubling","thermal-diamond-tester","specific-gravity","dispersion","birefringence"],"url":"https://digitalgemology.com/learn/glossary/moissanite","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report","https://digitalgemology.com/gemstones/moissanite"]},{"term":"Natural","slug":"natural","aka":["naturals"],"category":"Clarity characteristics","definition":"A natural is a part of the rough diamond's original surface left on the cut stone. It is usually on or near the girdle.","detail":"It is a blemish. It shows that the cutter kept as much weight as possible. It is also a clue to diamond. GIA lists it among the features that a diamond can show and moissanite doesn't.","on_report":"Drawn in green on GIA's clarity plot and listed after the inclusions in the key to symbols.","related":["indented-natural","girdle","blemish","key-to-symbols"],"url":"https://digitalgemology.com/learn/glossary/natural","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Natural diamond","slug":"natural-diamond","aka":["mined diamond","earth-grown diamond"],"category":"Treatments and origin","definition":"A natural diamond is a diamond that formed in the earth, about 160 km deep, under enormous heat and pressure. An eruption carried it to the surface.","detail":"A natural diamond and a laboratory-grown one are the same material. Only a gem lab's instruments can tell them apart. Screening devices answer Pass (natural) or Refer.","on_report":"GIA's Diamond Grading Report and Diamond Dossier are for natural diamonds.","related":["laboratory-grown-diamond","diamond","screening-device","diamond-grading-report"],"url":"https://digitalgemology.com/learn/glossary/natural-diamond","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin"]},{"term":"Near colorless (G–J)","slug":"near-colorless","aka":["G-J"],"category":"Color and fluorescence","definition":"Near colorless is the D-to-Z range from G to J.","detail":"A near-colorless diamond looks white face up. A slight tint shows only beside a better grade, most of all in larger stones. Trading down from colorless to near colorless is a common way to pay for size.","on_report":"Shown by the color letter. IGI names G–J as Near Colorless on its scale.","related":["colorless","faint-color","color-grade"],"url":"https://digitalgemology.com/learn/glossary/near-colorless","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z","https://digitalgemology.com/learn/four-cs/which-of-the-four-cs-matters-most"]},{"term":"Needle","slug":"needle","aka":["needles","rutile needle","needle inclusion"],"category":"Clarity characteristics","definition":"A needle is a thin, elongated crystal inclusion that looks like a tiny rod at 10x.","detail":"In corundum, needles are often rutile; crossing groups of them are called silk. In moissanite, GIA found thin white needles, roughly parallel, running down from the table along the optic axis. That pattern is a hint toward moissanite, not proof.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["crystal-inclusion","silk","moissanite","inclusion"],"url":"https://digitalgemology.com/learn/glossary/needle","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test","https://digitalgemology.com/gemstones/ruby/emerald-square"]},{"term":"Nick","slug":"nick","aka":["nicks"],"category":"Clarity characteristics","definition":"A nick is a small notch on a facet junction, with no clear depth at 10x. It is usually on the girdle edge or at the culet.","detail":"It is a blemish. The polish grader looks for nicks at 10x, with pits, polish lines and scratches.","on_report":"A blemish; it can count in the polish grade.","related":["chip","abrasion","polish","blemish"],"url":"https://digitalgemology.com/learn/glossary/nick","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence"]},{"term":"No indications of heating","slug":"no-indications-of-heating","aka":["NH","unheated","no heat"],"category":"Treatments and origin","definition":"'No indications of heating' (NH) is the standard lab wording for a corundum that shows no signs of heat treatment.","detail":"It is the lab's finding from the stone's inclusions and spectra, not a guarantee. Some heating conditions leave certain inclusions unchanged. 'Unheated' is the trade word for the same claim.","on_report":"Further information: no indications of heating (NH).","related":["indications-of-heating","heat-treatment","silk","origin-report"],"url":"https://digitalgemology.com/learn/glossary/no-indications-of-heating","sources":[]},{"term":"OBJ file","slug":"obj-file","aka":["OBJ"],"category":"Cutting and files","definition":"OBJ is a mesh format that almost every 3D program reads, including Blender, ZBrush, Maya and Rhino.","detail":"FreeCAD and Cinema 4D also open it. Gem Cut Studio can export a design as OBJ.","how_we_measure":"Our 3D files include OBJ. Our tools also read your own OBJ files.","related":["mesh","3d-model","gem-cut-studio"],"url":"https://digitalgemology.com/learn/glossary/obj-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Old European cut","slug":"old-european-cut","aka":["old European","OEC","European cut"],"category":"Cuts and shapes","definition":"An old European cut is an antique round diamond with 58 facets, from the 1870s to about 1930. The bruting machine made its round outline possible.","detail":"It keeps the old mine's facet plan on a round outline. It is thinner than the cushion-shaped cuts before it and has a small table; our model's table is 45%. Most antique round diamonds are old Europeans.","related":["old-mine-cut","bruting","tolkowsky-ideal-cut","round-brilliant"],"url":"https://digitalgemology.com/learn/glossary/old-european-cut","sources":["https://digitalgemology.com/learn/historic-cuts/old-european-cut-diamond","https://digitalgemology.com/cuts/old-european"]},{"term":"Old mine cut","slug":"old-mine-cut","aka":["old mine","old miner","old-mine brilliant"],"category":"Cuts and shapes","definition":"An old mine cut is an antique cushion-shaped diamond of the 1700s and 1800s. It has the Peruzzi's 58 facets, cut more evenly by hand.","detail":"The name comes from the old mines of India and Brazil; Brazil's deposits were found in the 1720s. Old mine stones have a soft, chunky look, with big flashes and dark areas between them. The cushion cut developed from it.","related":["peruzzi-cut","old-european-cut","cushion-cut","bruting"],"url":"https://digitalgemology.com/learn/glossary/old-mine-cut","sources":["https://digitalgemology.com/learn/historic-cuts/old-mine-cut-diamond","https://digitalgemology.com/cuts/old-mine"]},{"term":"Old single cut","slug":"old-single-cut","aka":["single cut","eight cut"],"category":"Cuts and shapes","definition":"The old single cut, or eight cut, has 8 facets above the girdle and 18 in all. They are a table, 8 crown facets, 8 pavilion facets and a culet.","detail":"It dates from the 1400s. Then the only way to shape diamond was to grind it with diamond powder on a wheel. Few facets give few sparkles. Cutters still use the plan for very small stones.","related":["mazarin-cut","rose-cut","round-brilliant","facet"],"url":"https://digitalgemology.com/learn/glossary/old-single-cut","sources":["https://digitalgemology.com/learn/historic-cuts/old-single-cut-diamond","https://digitalgemology.com/cuts/old-single"]},{"term":"Optic axis","slug":"optic-axis","aka":["c-axis"],"category":"Optics","definition":"An optic axis is a direction in a doubly refractive crystal along which light does not split into two rays. Seen along it, the stone behaves as if it were singly refractive.","detail":"In uniaxial stones, the optic axis runs along the crystal's c-axis. Cutters often set it square to the table: in moissanite this hides the doubling face-up. In ruby and sapphire, this orientation gives the best face-up color. So look at a stone from more than one direction before you call it singly refractive.","how_we_measure":"The simulator and the Refractometer set the optic axis square to the table. The Polariscope and Dichroscope let you tilt the stone from straight down the axis to across it. The Spectroscope looks across the axis.","related":["uniaxial","doubly-refractive","polariscope","dichroscope","interference-figure"],"url":"https://digitalgemology.com/learn/glossary/optic-axis","sources":["https://digitalgemology.com/about/how-we-measure/simulator","https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Optic sign","slug":"optic-sign","aka":["optically positive","optically negative"],"category":"Optics","definition":"The optic sign of a uniaxial stone tells which of its two rays is slower. It is positive when the extraordinary index is higher than the ordinary one, and negative when it is lower.","detail":"Moissanite and amethyst are uniaxial positive. Ruby, sapphire and emerald are uniaxial negative. A gemologist reads the sign with a conoscope and a first-order red plate.","how_we_measure":"The Polariscope reads the sign from the colors of its computed interference figure. Blue in the northeast quadrant means positive; blue in the northwest means negative.","related":["uniaxial","red-plate","conoscope","interference-figure"],"url":"https://digitalgemology.com/learn/glossary/optic-sign","sources":["https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/tools/polariscope"]},{"term":"Optical activity","slug":"optical-activity","aka":["optical rotation"],"category":"Optics","definition":"Optical activity is the turning of the plane of vibration of light as it passes through some crystals. Quartz, and so amethyst, turns it as light travels along the optic axis.","detail":"Because of it, a quartz slab cut across its axis stays light, in color, between crossed polarizers. It also gives quartz a bull's-eye interference figure with no cross at the center.","how_we_measure":"The Polariscope uses ρ(λ) = 7.19 λ² / (λ² − 0.0926283²)² degrees per mm, with λ in µm. That gives 21.8° per mm at 589 nm, after Chandrasekhar (1957).","related":["interference-figure","polariscope","polarized-light","optic-axis"],"url":"https://digitalgemology.com/learn/glossary/optical-activity","sources":["https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Ordinary ray","slug":"ordinary-ray","aka":["o-ray"],"category":"Optics","definition":"The ordinary ray (o-ray) is the ray in a uniaxial stone that vibrates across the optic axis. It has the same refractive index, ω or nₒ, in every direction.","detail":"The ordinary index ω is 2.648 in moissanite, 1.770 in ruby and sapphire, 1.583 in emerald and 1.544 in amethyst. With the table cut across the axis, the o-ray gives one of the two refractometer edges.","how_we_measure":"Our Dichroscope shows the o-ray color in one window when the dichroscope lines up with the stone. In ruby the o-ray is purplish red; in emerald it is yellowish green.","related":["extraordinary-ray","birefringence","optic-axis","pleochroism"],"url":"https://digitalgemology.com/learn/glossary/ordinary-ray","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/about/how-we-measure/refractometer","https://digitalgemology.com/about/how-we-measure/dichroscope"]},{"term":"Origin report","slug":"origin-report","aka":["country-of-origin report","geographic origin","origin determination"],"category":"Treatments and origin","definition":"An origin report gives a lab's opinion on the country or region where a gem was mined.","detail":"GIA offers origin reports for ruby, sapphire, emerald, Paraíba tourmaline, red spinel and alexandrite. The lab compares the stone's inclusions and trace-element chemistry with reference stones of known origin. Some deposits overlap, so a lab cannot give an origin for every stone. Origin can change the price a lot, so ask for a report that supports any origin claim.","on_report":"Geographic origin: a country or region, when the lab can give an opinion.","related":["kashmir-sapphire","three-phase-inclusion","no-indications-of-heating","colored-stone-report"],"url":"https://digitalgemology.com/learn/glossary/origin-report","sources":["https://digitalgemology.com/learn/grading-reports/what-is-a-diamond-grading-report"]},{"term":"Oval cut","slug":"oval-cut","aka":["oval","oval brilliant"],"category":"Cuts and shapes","definition":"An oval cut is an elongated round: the brilliant facet plan stretched along one axis.","detail":"It faces up larger than a round of the same weight. Ovals can show a bow-tie across the belly. GIA's oval guide says that many people prefer a length-to-width ratio of 1.30 to 1.50. Some oval designs qualify for the AGS Ideal Report by GIA.","on_report":"No GIA cut grade; measurements as length × width × depth.","related":["fancy-shape","bow-tie","length-to-width-ratio","ags-ideal-report"],"url":"https://digitalgemology.com/learn/glossary/oval-cut","sources":["https://digitalgemology.com/cuts/oval","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report","https://digitalgemology.com/learn/bow-tie/which-diamond-shapes-have-a-bow-tie"]},{"term":"Over the limit (OTL)","slug":"over-the-limit","aka":["OTL"],"category":"Gem testing","definition":"Over the limit (OTL) is the refractometer reading of a stone whose refractive index is higher than the contact liquid's. The scale stays dark down to the liquid line, so there is no number to read.","detail":"Diamond, moissanite and cubic zirconia all read over the limit with 1.81 liquid. So a standard refractometer cannot tell these three apart. Moissanite's double refraction does not show on it either.","how_we_measure":"Our Refractometer shows the OTL view: the scale stays dark down to a faint liquid line at 1.81. The liquid's own critical angle in a glass of 1.88 is 74.3°.","related":["refractometer","contact-liquid","shadow-edge","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/over-the-limit","sources":["https://digitalgemology.com/about/how-we-measure/refractometer","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Overall score","slug":"overall-score","aka":["Global","global performance","Cut Performance Index overall score"],"category":"Light performance","definition":"The overall score is one figure out of 100 that combines our light-performance scores for one cut in one gemstone. The Cut Performance Wheel calls it Global.","detail":"It compares cuts within one gemstone only; never compare scores across stones. The weights are fixed settings of our method, not measured constants. The T73 round brilliant in diamond scores 89.8.","how_we_measure":"Overall = 0.34 B + 0.20 F + 0.16 T + 0.10 S + 0.10 Y + 0.10 (100 − leakage). Here B is brilliance, F fire, T tilt, S scintillation and Y symmetry. The census stores the figure unrounded. The Cut Performance Index shows one decimal, and cut pages show whole numbers.","related":["cut-performance-index","brilliance","fire","tilt-light-performance","leakage","fixed-settings"],"url":"https://digitalgemology.com/learn/glossary/overall-score","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"Padparadscha","slug":"padparadscha","aka":["padparadscha sapphire","padparadshah"],"category":"Colored gemstones","definition":"Padparadscha is a sapphire of soft pinkish orange to orangey pink. Its tones are pastel and its saturation low to medium.","detail":"The name refers to the lotus flower. Under the LMHC definition, it can come from any country, and heated stones keep the name. Diffusion, glass filling, irradiation, dye and coating rule the name out. A color stability test is mandatory: a stone that shifts out of range, for example to pink, loses the name.","on_report":"Identification: (natural) corundum. Species: padparadscha (sapphire). A stone that fails the stability test is named pink sapphire or fancy sapphire.","related":["fancy-sapphire","sapphire","stability","beryllium-diffusion","heat-treatment"],"url":"https://digitalgemology.com/learn/glossary/padparadscha","sources":[]},{"term":"Painting and digging","slug":"painting-and-digging","aka":["painting","digging out","painted girdle","dug-out girdle"],"category":"Anatomy and proportions","definition":"Painting and digging out are small tilts of the half facets along the girdle. Cutters use them to save weight.","detail":"GIA measures the degree of painting or digging and counts it in the cut grade. In our Cut Grading Tool, which reads GIA's system, Moderate gives at most Very Good, Significant Good and Severe Fair. Heavy painting on the pavilion breaks the pattern into broad dark zones. Heavy digging can drain contrast and leave a grayish stone.","on_report":"Not a separate line on GIA's report, but it can limit the cut grade.","related":["upper-half-facet","lower-half-facet","magic-sizes","cut-grade","weight-ratio"],"url":"https://digitalgemology.com/learn/glossary/painting-and-digging","sources":["https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length","https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined"]},{"term":"Pavilion","slug":"pavilion","aka":["bottom","pavilion of a diamond"],"category":"Anatomy and proportions","definition":"The pavilion is the cone-shaped part of a cut stone below the girdle. A standard round brilliant has 24 pavilion facets: 8 mains and 16 lower halves.","detail":"The pavilion works like a mirror: it reflects light that enters through the top back up through the crown. If it is too shallow or too steep, light escapes through the pavilion facets.","on_report":"Described on the proportion diagram by the pavilion angle and the pavilion depth.","related":["pavilion-angle","pavilion-depth","pavilion-main","lower-half-facet","culet","leakage"],"url":"https://digitalgemology.com/learn/glossary/pavilion","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-pavilion-angle"]},{"term":"Pavilion angle","slug":"pavilion-angle","aka":["pavilion angle (degrees)"],"category":"Anatomy and proportions","definition":"The pavilion angle is the slope of the pavilion mains, measured against the plane of the girdle.","detail":"It decides how much light the pavilion reflects back up, and it is the most sensitive number on a round. Tolkowsky calculated 40.75° in 1919, and GIA's Excellent limit for the pavilion alone is 40.6° to 41.8°. In our Cut Grading Tool, at a 53% table and a 34.5° crown, Excellent runs only from 40.6° to 41.4°.","on_report":"On the proportion diagram, in degrees, in 0.2° steps. IGI's Ideal band is 40.0 to 41.3°.","related":["pavilion-depth","pavilion-main","crown-angle","critical-angle","leakage"],"url":"https://digitalgemology.com/learn/glossary/pavilion-angle","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-pavilion-angle","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"Pavilion depth","slug":"pavilion-depth","category":"Anatomy and proportions","definition":"Pavilion depth is the depth of the pavilion, from the girdle to the culet. It is a percentage of the average diameter.","detail":"For a pointed culet it follows from the pavilion angle alone: half the diameter × the tangent of the angle. At 40.8° it is about 43%.","on_report":"On the proportion diagram as a percentage. IGI's Ideal band is 42 to 44%.","related":["pavilion-angle","total-depth","pavilion"],"url":"https://digitalgemology.com/learn/glossary/pavilion-depth","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-pavilion-angle","https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained"]},{"term":"Pavilion main","slug":"pavilion-main","aka":["pavilion mains","pavilion main facets","lower main facets"],"category":"Anatomy and proportions","definition":"Pavilion mains are the 8 long facets of a round brilliant's pavilion that run from the girdle to the culet.","detail":"The pavilion angle is measured on the mains. In a hearts and arrows viewer, each arrow's shaft is a pavilion main, reflected in the main opposite it.","on_report":"Their slope is the pavilion angle on the proportion diagram.","related":["pavilion-angle","lower-half-facet","hearts-and-arrows","pavilion"],"url":"https://digitalgemology.com/learn/glossary/pavilion-main","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-pavilion-angle","https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length"]},{"term":"Pear cut","slug":"pear-cut","aka":["pear","pear shape"],"category":"Cuts and shapes","definition":"A pear cut is a brilliant with one rounded end and one point, halfway between an oval and a marquise.","detail":"Many pears show a bow-tie. GIA's pear guide says it gets darker as the difference between length and width grows. Our Pear design needs a 13.8° spread of pavilion angles around its outline.","on_report":"No GIA cut grade; measurements as length × width × depth.","related":["fancy-shape","bow-tie","marquise-cut","oval-cut"],"url":"https://digitalgemology.com/learn/glossary/pear-cut","sources":["https://digitalgemology.com/cuts/pear","https://digitalgemology.com/learn/bow-tie/which-diamond-shapes-have-a-bow-tie"]},{"term":"Peruzzi cut","slug":"peruzzi-cut","aka":["Peruzzi","triple cut","triple-cut brilliant"],"category":"Cuts and shapes","definition":"The Peruzzi, or triple cut, is a cushion brilliant of the late 1600s with 58 facets. Its 32-facet crown has 8 bezels, 8 stars and 16 upper halves.","detail":"Its crown plan is the crown of every modern round. It is attributed to the Venetian polisher Vincent Peruzzi. It is still a deep cushion, cut entirely by hand.","related":["mazarin-cut","old-mine-cut","bezel-facet","upper-half-facet"],"url":"https://digitalgemology.com/learn/glossary/peruzzi-cut","sources":["https://digitalgemology.com/learn/historic-cuts/peruzzi-cut-diamond","https://digitalgemology.com/cuts/peruzzi"]},{"term":"Pigeon's blood","slug":"pigeons-blood","aka":["pigeon blood red","pigeonblood"],"category":"Colored gemstones","definition":"Pigeon's blood is a trade name for a ruby of intense, saturated, even red. It has no single agreed standard.","detail":"The term once described fine rubies from Mogok, Burma. Gübelin and SSEF agreed in 2015 to harmonize their criteria, but interpretations still differ. Gübelin also asks for strong fluorescence and excludes any treatment, heating included. Other labs that use the term set their own criteria.","on_report":"Some labs add it as a color term on a report or appendix, each under its own criteria.","related":["ruby","royal-blue","trade-color-term","fluorescence","saturation"],"url":"https://digitalgemology.com/learn/glossary/pigeons-blood","sources":[]},{"term":"Pink sapphire","slug":"pink-sapphire","category":"Colored gemstones","definition":"Pink sapphire is corundum whose color is pink rather than red. Chromium causes both its pink and the red of ruby.","detail":"GIA's master stones require red to be the dominant hue before a stone is called ruby. Judging the dominant hue is subjective, so a stone near the line can get either name. Sri Lanka has traditionally called pink stones ruby. A lab report states the name that lab gives.","on_report":"Variety: pink sapphire.","related":["ruby","sapphire","fancy-sapphire","hue","saturation"],"url":"https://digitalgemology.com/learn/glossary/pink-sapphire","sources":[]},{"term":"Pinpoint","slug":"pinpoint","aka":["pinpoints"],"category":"Clarity characteristics","definition":"A pinpoint is a very small crystal inside a diamond that looks like a tiny dot at 10x.","detail":"In our simulator, a single pinpoint is all a VVS1 stone shows. Many pinpoints grouped together form a cloud.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["cloud","crystal-inclusion","vvs","inclusion"],"url":"https://digitalgemology.com/learn/glossary/pinpoint","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Pit","slug":"pit","aka":["pits"],"category":"Clarity characteristics","definition":"A pit is a small opening in the surface that looks like a tiny white dot.","detail":"It is a blemish. The polish grader looks for pits at 10x, with nicks, polish lines and scratches.","on_report":"A blemish; it can count in the polish grade.","related":["nick","polish","blemish"],"url":"https://digitalgemology.com/learn/glossary/pit","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence"]},{"term":"Planar facet","slug":"planar-facet","aka":["flat facet"],"category":"Cutting and files","definition":"A planar facet is a facet that lies exactly in one flat plane. Real cut stones and our models have planar facets; curved or smoothed display models do not.","detail":"A gem refracts correctly only when its facets are truly flat. A flat facet keeps a beam together, and a curved one spreads it. So a rounded, smoothed or scanned model cannot be measured as a cut stone.","how_we_measure":"Our tools group a model's triangles into facets by the plane they lie on. A model that breaks into more than 2,000 flat pieces counts as a scan or a smooth shape. The tools do not measure it as a cut.","related":["facet","mesh","convex","measure-your-own-stone"],"url":"https://digitalgemology.com/learn/glossary/planar-facet","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/your-own-file","https://digitalgemology.com/about/how-we-measure/3d-models"]},{"term":"Pleochroism","slug":"pleochroism","aka":["dichroism","trichroism","dichroic"],"category":"Optics","definition":"Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.","detail":"A uniaxial stone can show two colors (dichroism); a biaxial stone can show three (trichroism). Ruby shows purplish red and orangy red; sapphire violetish blue and greenish blue. Emerald shows yellowish green and bluish green. Colorless doubly refractive stones, such as moissanite, show no pleochroism.","how_we_measure":"The Dichroscope computes the two colors from measured polarized absorption spectra. It classes the color difference ΔE00 as none (under 1), weak (1 to 5), distinct (5 to 16.5) or strong. The simulator's pleochroism is tuned to each stone's published pair, not measured.","related":["dichroscope","ordinary-ray","extraordinary-ray","ciede2000","biaxial"],"url":"https://digitalgemology.com/learn/glossary/pleochroism","sources":["https://digitalgemology.com/about/how-we-measure/dichroscope","https://digitalgemology.com/tools/dichroscope","https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"Polariscope","slug":"polariscope","aka":["gem polariscope"],"category":"Gem testing","definition":"A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.","detail":"A singly refractive stone stays dark as it turns. A doubly refractive stone blinks: four light and four dark positions per turn. An aggregate stays light at every turn. Down its optic axis, a doubly refractive stone also stays dark, so test more than one direction.","how_we_measure":"Our Polariscope computes the view by Jones calculus at 81 wavelengths, from each stone's refractive indices and measured absorption. It also has a conoscope, a red plate and an illustrative diamond strain. It is a teaching model, not a test of a real stone.","related":["blink-polariscope","strain","conoscope","red-plate","singly-refractive","doubly-refractive"],"url":"https://digitalgemology.com/learn/glossary/polariscope","sources":["https://digitalgemology.com/tools/polariscope","https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Polarized light","slug":"polarized-light","aka":["polarization"],"category":"Optics","definition":"Polarized light vibrates in one direction across its path, instead of in every direction. A polarizing filter passes only the light that vibrates along its own direction.","detail":"Two filters at right angles (crossed) pass no light, unless something between them changes the light. Many phone and laptop screens give out polarized light. So never test a polariscope or dichroscope against a screen.","how_we_measure":"The Polariscope models ideal polarizers by Jones calculus. Our light-performance tools average Fresnel losses over polarization. Only the simulator's render follows polarization from bounce to bounce, in doubly refractive stones.","related":["polariscope","doubly-refractive","jones-calculus","fresnel-reflection"],"url":"https://digitalgemology.com/learn/glossary/polarized-light","sources":["https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/about/how-we-measure/limits"]},{"term":"Polish","slug":"polish","aka":["polish grade"],"category":"Grading report","definition":"Polish is the grade for the smoothness of the facet surfaces, from Excellent to Poor. The grader looks at 10x for pits, nicks, polish lines, scratches, burn marks and rough patches.","detail":"On a GIA round, polish limits the cut grade. Excellent or Very Good polish allows Excellent, Good allows at most Very Good, Fair allows Good, and Poor gives Poor. Polish doesn't say where the facets point, so it can't predict a bow-tie.","on_report":"In the additional grading section of GIA and IGI reports. IGI gives a polish grade for every shape and color.","related":["symmetry","cut-grade","polish-lines","burn-mark","triple-excellent"],"url":"https://digitalgemology.com/learn/glossary/polish","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence","https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report"]},{"term":"Polish lines","slug":"polish-lines","aka":["polish marks","polishing lines"],"category":"Clarity characteristics","definition":"Polish lines are fine parallel grooves and ridges left by polishing. They can occur on any facet but don't cross facet junctions, and they look transparent or white.","detail":"A diamond's cutter works each facet in its own direction, because diamond is harder in some directions than in others. Polish lines that run the same way on facet after facet are a clue to moissanite, not proof.","on_report":"A blemish; polish lines count in the polish grade and can stop a stone from being Flawless.","related":["polish","flawless","burn-mark","facet-doubling","blemish"],"url":"https://digitalgemology.com/learn/glossary/polish-lines","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test","https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Princess cut","slug":"princess-cut","aka":["princess"],"category":"Cuts and shapes","definition":"A princess cut is a square brilliant with sharp corners and chevron-patterned pavilion facets.","detail":"Its corners are its weak points, so settings usually protect them with V-prongs. Some princess designs qualify for the AGS Ideal Report by GIA.","on_report":"No GIA cut grade.","related":["fancy-shape","cushion-cut","radiant-cut","ags-ideal-report"],"url":"https://digitalgemology.com/learn/glossary/princess-cut","sources":["https://digitalgemology.com/cuts/princess","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report"]},{"term":"Proportion diagram","slug":"proportion-diagram","aka":["proportions diagram","profile diagram","graphic of proportions"],"category":"Grading report","definition":"The proportion diagram is a side-view profile of the stone, drawn to its measured proportions. It shows the table, depth, crown, pavilion, star and lower half lengths, girdle and culet.","detail":"An optical device measures the stone without touching it, and the report gives the numbers in rounded steps. GIA designed its cut grade so that stones with the same rounded proportions and descriptions get the same grade. So you can estimate the cut grade of a round from this diagram.","on_report":"On GIA's Diamond Grading Report and Dossier. IGI measures the stone with a 3D optical scan. Its report prints the key measurements and can add a graphic.","related":["table-size","total-depth","crown-angle","pavilion-angle","girdle-thickness","culet-size"],"url":"https://digitalgemology.com/learn/glossary/proportion-diagram","sources":["https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"Proportions","slug":"proportions","aka":["diamond proportions","make"],"category":"Anatomy and proportions","definition":"A diamond's proportions are the angles and percentages that describe its shape in profile. They cover the table, crown, pavilion, star and lower half lengths, girdle, culet and total depth.","detail":"Each proportion is an angle or a percentage of the average diameter. So you can compare stones of any size directly. The proportions set how a stone handles light and most of its cut grade. Two values that are each in range can still lower the grade as a pair.","on_report":"On the proportion diagram of GIA and IGI reports.","related":["proportion-diagram","average-diameter","total-depth","crown-angle","pavilion-angle","cut-grade"],"url":"https://digitalgemology.com/learn/glossary/proportions","sources":["https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions","https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram"]},{"term":"Quartz","slug":"quartz","aka":["silicon dioxide"],"category":"Colored gemstones","definition":"Quartz is the mineral species of amethyst and citrine. It is silicon dioxide in a trigonal crystal.","detail":"Quartz has a refractive index of 1.544 to 1.553, a specific gravity of 2.65 and a hardness of 7. Its low refractive index needs a steep pavilion. Quartz turns the plane of polarized light, so its conoscope figure is a bull's-eye. Laboratories grow quartz only by the hydrothermal method.","on_report":"Labs print 'quartz' as the identification and the variety, such as amethyst or citrine.","related":["amethyst","citrine","ametrine","hydrothermal-growth","gem-species"],"url":"https://digitalgemology.com/learn/glossary/quartz","sources":["https://digitalgemology.com/gemstones/amethyst","https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Radiant cut","slug":"radiant-cut","aka":["radiant"],"category":"Cuts and shapes","definition":"A radiant cut has the outline of an emerald cut, with cropped corners, and a brilliant facet plan.","detail":"It combines the clean outline of a step cut with the sparkle of a brilliant. Mixed facets also help spread color evenly in colored stones.","on_report":"No GIA cut grade.","related":["emerald-cut","princess-cut","brilliant-cut"],"url":"https://digitalgemology.com/learn/glossary/radiant-cut","sources":["https://digitalgemology.com/cuts/radiant"]},{"term":"Raman spectroscopy","slug":"raman-spectroscopy","aka":["Raman"],"category":"Gem testing","definition":"Raman spectroscopy is a laboratory method that identifies a material from how it scatters laser light. Diamond shows a peak at 1332 cm⁻¹.","detail":"Moissanite's Raman spectrum lacks that peak. With infrared spectra and X-ray fluorescence, which finds silicon, it identifies moissanite submitted as diamond.","related":["screening-device","absorption-spectrum"],"url":"https://digitalgemology.com/learn/glossary/raman-spectroscopy","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Ray tracing","slug":"ray-tracing","aka":["ray trace"],"category":"How we measure","definition":"Ray tracing follows straight lines of light through a model of a stone. At each facet it applies refraction and reflection, and it records where the light comes from or where it leaves.","detail":"Ray tracing describes the geometry of a cut and the optics of its material. Each tracer leaves out some physics for speed, so results from different tracers differ.","how_we_measure":"Our tools trace light in four ways. The simulator traces per pixel, with up to 64 samples and 14 bounces. The Brightness & Tilt Analysis uses a 132 × 132 grid and 12 bounces. The Cut Grading Tool's brilliance uses an 84 × 84 grid and 12 bounces. The light census uses 3,200 seeded rays and 12 bounces. All four treat total internal reflection as lossless.","related":["light-census","snells-law","fresnel-reflection","total-internal-reflection","3d-gemstone-simulator"],"url":"https://digitalgemology.com/learn/glossary/ray-tracing","sources":["https://digitalgemology.com/about/how-we-measure/light-measures","https://digitalgemology.com/about/how-we-measure","https://digitalgemology.com/about/how-we-measure/limits"]},{"term":"Reference size","slug":"reference-size","category":"Cutting and files","definition":"The reference size is the size at which a 3D model is delivered, in millimeters. Our cut pages give it, with the model's weight at that size in each gemstone.","detail":"The T73's reference size is 8.15 × 8.15 × 5.03 mm, which weighs 1.99 ct in diamond. You can scale a model uniformly to any size. The weight follows the cube of the size.","how_we_measure":"Each build measures size, length to width and depth from the mesh, with the stone level in plan. It checks them against the STEP solid.","related":["weight-from-volume","3d-model","specific-gravity"],"url":"https://digitalgemology.com/learn/glossary/reference-size","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"Refractive index","slug":"refractive-index","aka":["RI","index of refraction"],"category":"Optics","definition":"Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.","detail":"Diamond's index is 2.417. Moissanite reads 2.648 to 2.691 and cubic zirconia 2.15 to 2.18. Ruby and sapphire read 1.762 to 1.770, emerald 1.577 to 1.583 and amethyst 1.544 to 1.553. The index sets the critical angle, so a design for diamond does not work the same way in another stone.","how_we_measure":"Our gemstone pages give each stone's index at the sodium line, 589 nm. For angle limits we use the lower end of a range, because it gives the stricter limit. On the Refractometer, diamond, moissanite and cubic zirconia read over the limit.","related":["critical-angle","dispersion","birefringence","refractometer","snells-law"],"url":"https://digitalgemology.com/learn/glossary/refractive-index","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-refractive-index-and-cut","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Refractometer","slug":"refractometer","aka":["gem refractometer","critical angle refractometer"],"category":"Gem testing","definition":"A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.","detail":"The reading is where a shadow edge falls on the scale. A standard instrument has a glass of about 1.88 and 1.81 liquid, so it cannot read stones above 1.81. A doubly refractive stone gives two edges; their gap is the birefringence. Real readings have a tolerance of about 0.005 on a well-polished facet.","how_we_measure":"Our Refractometer models a standard instrument: glass 1.88, liquid 1.81 and yellow light at 589 nm. The Fresnel equations, for both polarizations, light each point of the scale. It is a teaching model, not a test of a real stone.","related":["shadow-edge","over-the-limit","contact-liquid","refractive-index","birefringence"],"url":"https://digitalgemology.com/learn/glossary/refractometer","sources":["https://digitalgemology.com/tools/refractometer","https://digitalgemology.com/about/how-we-measure/refractometer"]},{"term":"Report Check","slug":"report-check","aka":["GIA Report Check","Verify Your Report","QR code","report verification"],"category":"Grading report","definition":"Report Check is GIA's online database. Enter a report number to see what GIA itself recorded; IGI's equivalent is Verify Your Report.","detail":"Scan the QR code on the report or type the number. For digital reports, the online version is the definitive one. Then check the stone against the record: weight, measurements and inscription.","on_report":"GIA reports carry a QR code that leads to the online record. IGI reports carry one too.","related":["report-number","inscription","security-features","measurements"],"url":"https://digitalgemology.com/learn/glossary/report-check","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Report date","slug":"report-date","aka":["date","date of examination"],"category":"Grading report","definition":"The report date is the day the lab examined the stone. Everything on the report is true for that day.","detail":"A report describes the stone as it was then. Later damage, a new treatment or a recut is not on it.","on_report":"The first line of the identity block on a GIA report.","related":["report-number","diamond-grading-report"],"url":"https://digitalgemology.com/learn/glossary/report-date","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section"]},{"term":"Report number","slug":"report-number","aka":["certificate number","GIA report number","IGI report number"],"category":"Grading report","definition":"The report number is the unique number a lab gives a report. GIA registers it in its database, so anyone can look it up.","detail":"Many stones carry their report number on the girdle, written by a laser. Compare that number, the online record and the stone's weight and measurements. A matching number alone proves little. GIA has found moissanites with a fake GIA inscription that copied a real report number.","on_report":"In the identity block near the top of the report. IGI inscribes every laboratory-grown stone with \"LG\" before its report number.","related":["inscription","report-check","measurements","diamond-grading-report"],"url":"https://digitalgemology.com/learn/glossary/report-number","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Residues in healed fissures","slug":"residues-in-healed-fissures","aka":["flux healing","FAH"],"category":"Treatments and origin","definition":"Residues in healed fissures are remains of the heating process, often glassy. They stay in fissures that healed while a corundum was heated.","detail":"LMHC labs rate the amount as minor, moderate or significant. The residue ranges from fine 'bubbles' to thick films. Glass-filled cavities can also be present. Lead-glass filling is a different treatment, with its own wording.","on_report":"Indications of heating, with residues rated FAH 1 (minor), FAH 2 (moderate) or FAH 3 (significant). Filled cavities get their own code, such as FAH C3.","related":["indications-of-heating","heat-treatment","fingerprint","lead-glass-filling"],"url":"https://digitalgemology.com/learn/glossary/residues-in-healed-fissures","sources":[]},{"term":"Resin filling","slug":"resin-filling","aka":["polymer filling","artificial resin"],"category":"Treatments and origin","definition":"Resin filling is filling a gem's fissures or cavities with a resin, often one that hardens, to hide them.","detail":"It is common in emerald. Resin can deteriorate over time. A colored resin also changes the color, and the lab then reports a colored filler.","on_report":"Fissure filling with its amount (F1 to F3), or cavity filling (C1 to C3). A colored filler is reported as such.","related":["clarity-enhancement","emerald-oiling","manufactured-product","enhancement-degree"],"url":"https://digitalgemology.com/learn/glossary/resin-filling","sources":[]},{"term":"Rose cut","slug":"rose-cut","aka":["rose"],"category":"Cuts and shapes","definition":"A rose cut is a dome of triangular facets on a flat base: all crown and no pavilion.","detail":"It spread from the middle of the 1500s. It suited flat rough and lost little weight. Light leaves at the same angle it enters, so the colors recombine: a rose shows almost no fire.","related":["brilliant-cut","mazarin-cut","old-single-cut","fire"],"url":"https://digitalgemology.com/learn/glossary/rose-cut","sources":["https://digitalgemology.com/learn/historic-cuts/mazarin-cut-diamond"]},{"term":"Rough girdle","slug":"rough-girdle","category":"Clarity characteristics","definition":"A rough girdle has an irregular or granular girdle surface.","detail":"Do not confuse it with a bruted girdle, which is left with the surface from rounding. A diamond's girdle can look waxy to granular; in GIA's 1997 study, moissanite girdles were frosted, polished or striated.","on_report":"A blemish.","related":["girdle","bearded-girdle","bruting","blemish"],"url":"https://digitalgemology.com/learn/glossary/rough-girdle","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test"]},{"term":"Round brilliant","slug":"round-brilliant","aka":["standard round brilliant","round brilliant cut","RBC","SRB","round"],"category":"Cuts and shapes","definition":"A round brilliant is a round diamond with the brilliant facet plan. It has 33 crown and 24 pavilion facets: 57 in all, or 58 with a culet.","detail":"It is the most common shape. GIA gives a cut grade only to standard round brilliants in the D-to-Z range. Most are now finished with a faceted girdle, which is counted separately. Its proportions go back to Tolkowsky's 1919 calculation.","on_report":"Written \"Round Brilliant\" in the shape and cutting style line.","related":["brilliant-cut","tolkowsky-ideal-cut","cut-grade","facet","fancy-shape"],"url":"https://digitalgemology.com/learn/glossary/round-brilliant","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/grading-reports/diamond-report-grades","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"Royal blue","slug":"royal-blue","category":"Colored gemstones","definition":"Royal blue is a trade name for a sapphire of intense, saturated, even blue. It is not a single agreed standard.","detail":"The term is linked to Burma. Gübelin also gives it to sapphires from other deposits, such as Madagascar and Sri Lanka. Gübelin and SSEF agreed in 2015 to harmonize their criteria, and other labs use their own. Gübelin excludes any treatment, heating included.","on_report":"Some labs add it as a color term on a report or appendix, each under its own criteria.","related":["sapphire","pigeons-blood","kashmir-sapphire","trade-color-term","saturation"],"url":"https://digitalgemology.com/learn/glossary/royal-blue","sources":[]},{"term":"Ruby","slug":"ruby","aka":["red corundum"],"category":"Colored gemstones","definition":"Ruby is the red variety of the mineral corundum. Chromium causes its red color.","detail":"Ruby has the same refractive index (1.762 to 1.770), hardness (9) and specific gravity (4.00) as sapphire. Red must be the dominant hue before GIA calls a stone ruby; a stone too light may be pink sapphire. Corundum has excellent toughness and no cleavage, so ruby suits daily wear.","on_report":"Labs identify it as (natural) corundum, species or variety ruby. Any treatment, such as indications of heating or glass in fissures, goes under Further information or Comments.","related":["corundum","sapphire","pink-sapphire","pigeons-blood","heat-treatment","silk"],"url":"https://digitalgemology.com/learn/glossary/ruby","sources":["https://digitalgemology.com/gemstones/ruby","https://digitalgemology.com/tools/dichroscope","https://digitalgemology.com/tools/spectroscope","https://digitalgemology.com/tools/polariscope","https://digitalgemology.com/tools/refractometer"]},{"term":"Sapphire","slug":"sapphire","aka":["blue sapphire"],"category":"Colored gemstones","definition":"Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.","detail":"Iron and titanium cause the blue, and only a few hundredths of a percent of them are enough. Sapphire has the same optics, hardness and toughness as ruby. Many sapphires show color zoning, so the orientation of the table matters.","on_report":"Labs identify it as (natural) corundum, variety sapphire. A sapphire of another color is named by its color, for example pink sapphire. Treatments go under Further information or Comments.","related":["corundum","ruby","fancy-sapphire","royal-blue","color-zoning","asterism"],"url":"https://digitalgemology.com/learn/glossary/sapphire","sources":["https://digitalgemology.com/gemstones/sapphire","https://digitalgemology.com/tools/dichroscope","https://digitalgemology.com/tools/spectroscope","https://digitalgemology.com/gemstones/sapphire/emerald-cut"]},{"term":"Saturation","slug":"saturation","aka":["color saturation","intensity"],"category":"Colored gemstones","definition":"Saturation is the strength or purity of a gem's color, from grayish or brownish to vivid.","detail":"Strong saturation is preferred in ruby, sapphire and emerald. Brownish tints lower the value of amethyst sharply. Padparadscha is the exception: its definition asks for low to medium saturation.","related":["hue","tone","padparadscha","pigeons-blood","royal-blue"],"url":"https://digitalgemology.com/learn/glossary/saturation","sources":["https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence"]},{"term":"SC2 lighting","slug":"sc2-lighting","aka":["SC2 dome"],"category":"How we measure","definition":"SC2 is a dome model with a ring of light that peaks at 45° elevation. It is dark overhead and at the horizon.","detail":"Of our three dome models, it is the closest to the light on a stone on a hand. The definition follows Robert W. Strickland's GemRay user guide; the implementation is our own.","how_we_measure":"In the Brightness & Tilt Analysis, W = 2 sin e cos e for elevation e. W is 0 inside the head shadow and below the horizon.","related":["dome-model","iso-lighting","cos-lighting","brightness-and-tilt-analysis"],"url":"https://digitalgemology.com/learn/glossary/sc2-lighting","sources":["https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/tools/brightness-tilt-analysis"]},{"term":"Schlick's approximation","slug":"schlick-approximation","aka":["Schlick Fresnel"],"category":"How we measure","definition":"Schlick's approximation is a short formula for Fresnel reflectance: R(θ) = F0 + (1 − F0)(1 − cos θ)^5. F0 is the reflectance at normal incidence, ((n − 1)/(n + 1))².","detail":"It is fast and close to the full Fresnel equations for unpolarized light. In our engine, F0 is 0.172 for diamond and 0.204 for moissanite.","how_we_measure":"The Brightness & Tilt Analysis and the Cut Grading Tool's brilliance apply it at the entry and exit facets. The simulator uses the full Fresnel equations instead.","related":["fresnel-reflection","brightness-and-tilt-analysis","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/schlick-approximation","sources":["https://digitalgemology.com/about/how-we-measure/brightness-tilt","https://digitalgemology.com/about/how-we-measure/gemstone-constants"]},{"term":"Scintillation","slug":"scintillation","aka":["sparkle"],"category":"Light performance","definition":"Scintillation is the pattern of light and dark flashes a stone shows as it, the light or the viewer moves. Our score reads it from the number of facets that reach the surface of the cut.","detail":"Our scintillation score is not traced. It rises with facet count and levels off, so it is the same in every gemstone. The T73, with 73 facets, scores 80; the T57, with 57 facets, scores 72. A high score means many facets, not more light.","how_we_measure":"S = 100 × (1 − e^(−f/45)), where f is the number of facets that reach the surface. Buried planes do not count. The Cut Performance Wheel, the cut pages and the Cut Performance Index show it. The Cut Grading Tool uses a different, modeled scintillation that depends on crown and pavilion angle.","related":["brilliance","fire","facet-count","cut-performance-wheel","fixed-settings"],"url":"https://digitalgemology.com/learn/glossary/scintillation","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/cut-grading-tool"]},{"term":"Scratch","slug":"scratch","aka":["scratches"],"category":"Clarity characteristics","definition":"A scratch is a thin, dull white line across the surface with no clear depth at 10x.","detail":"Diamond, at Mohs 10, resists scratches best of all gems, but diamonds can scratch each other. So keep diamond jewelry apart from other diamonds.","on_report":"A blemish; it can count in the polish grade.","related":["polish","hardness","abrasion","blemish"],"url":"https://digitalgemology.com/learn/glossary/scratch","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence"]},{"term":"Screening device","slug":"screening-device","aka":["diamond screener","Pass/Refer"],"category":"Gem testing","definition":"A screening device separates natural diamonds from laboratory-grown diamonds and simulants. Most answer Pass (natural) or Refer (send to a laboratory).","detail":"GIA's DiamondCheck and iD100 are examples. A referred stone may still be natural. Depending on how it tests, a screener may pass a simulant, so know what your device covers.","related":["thermal-diamond-tester","electrical-conductivity-tester","raman-spectroscopy"],"url":"https://digitalgemology.com/learn/glossary/screening-device","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Security features","slug":"security-features","aka":["report security features"],"category":"Grading report","definition":"Security features are the marks on a printed report that make it hard to copy. On a GIA report they are microprinting, security screens, watermarks, a two-dimensional barcode, a hologram and sheet numbering.","detail":"They protect the paper, not the link between paper and stone. Always check the stone itself against the report too.","on_report":"Printed into GIA's paper report. IGI says that its paper report has at least five security features.","related":["report-check","report-number","inscription"],"url":"https://digitalgemology.com/learn/glossary/security-features","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section"]},{"term":"Shadow edge","slug":"shadow-edge","aka":["refractometer reading"],"category":"Gem testing","definition":"The shadow edge is the line between the dark and the bright part of a refractometer scale. It falls at the refractive index of the stone.","detail":"Above the edge the scale is dark, and below it bright. A doubly refractive stone shows two edges, with a half-bright band between them. A singly refractive stone shows one edge, which stays in place as the stone turns.","how_we_measure":"Our Refractometer places each edge exactly at the published index. Amethyst, for example, shows edges at 1.544 and 1.553.","related":["refractometer","over-the-limit","birefringence","refractive-index"],"url":"https://digitalgemology.com/learn/glossary/shadow-edge","sources":["https://digitalgemology.com/about/how-we-measure/refractometer","https://digitalgemology.com/tools/refractometer"]},{"term":"Shape and cutting style","slug":"shape-and-cutting-style","aka":["shape","cutting style"],"category":"Grading report","definition":"Shape and cutting style is the report line for the outline of the stone and the pattern of its facets. An example is \"Round Brilliant\".","detail":"Shape is the outline: round, oval, pear and so on. The cutting style is the facet pattern, such as brilliant or step cut, not its quality. The cut grade is a different thing: how well the proportions and finish work.","on_report":"In the identity block, after the report number.","related":["round-brilliant","fancy-shape","brilliant-cut","step-cut","cut-grade"],"url":"https://digitalgemology.com/learn/glossary/shape-and-cutting-style","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section","https://digitalgemology.com/learn/four-cs/diamond-cut-grade-excellent-vs-poor"]},{"term":"Shape-norm","slug":"shape-norm","aka":["shape-normalized return"],"category":"Light performance","definition":"Shape-norm compares a cut's brilliance with a ceiling for its shape family. It shows how close a cut comes to what cuts of its family reach, rather than to a round.","detail":"The ceilings are 93 for round brilliants, 86 for fancy brilliants, 76 for step cuts and 64 for other cuts. The score is capped at 125, so it can go above 100.","how_we_measure":"Shape-norm = min(125, 100 × B ÷ C). B is census brilliance, and C is the ceiling of the cut's family. For your own file, the Wheel sorts the stone into a round brilliant, a step cut or a fancy brilliant.","related":["brilliance","cut-performance-wheel","fixed-settings","light-census"],"url":"https://digitalgemology.com/learn/glossary/shape-norm","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Shoulder","slug":"shoulder","aka":["shoulders"],"category":"Anatomy and proportions","definition":"The shoulder of an elongated stone is where the outline curves away from the end.","detail":"GIA's oval guide uses head or end, shoulder and belly to name the parts of an elongated stone. On DG-Bowtie, the dark band across the belly stops near the shoulders, where the pavilion facets are less steep.","related":["head","belly","oval-cut","pear-cut"],"url":"https://digitalgemology.com/learn/glossary/shoulder","sources":["https://digitalgemology.com/learn/bow-tie/bow-tie-effect-in-diamonds","https://digitalgemology.com/learn/bow-tie/how-to-fix-a-bow-tie"]},{"term":"Silk","slug":"silk","aka":["rutile silk"],"category":"Clarity characteristics","definition":"Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.","detail":"A little silk scatters light and spreads color over dark areas, which can soften and even out the color. Too much silk lowers transparency and value. Intact silk shows that a stone was not heated to very high temperatures, but not that it was never heated.","on_report":"Not listed as a grade. Its state helps a lab decide between 'no indications of heating' and 'indications of heating'.","related":["needle","asterism","no-indications-of-heating","heat-treatment","ruby"],"url":"https://digitalgemology.com/learn/glossary/silk","sources":["https://digitalgemology.com/gemstones/ruby/emerald-square"]},{"term":"Simulant","slug":"simulant","aka":["diamond simulant","imitation","look-alike","substitute"],"category":"Diamond and simulants","definition":"A simulant is a material that looks like a gem but has a different chemistry, structure and properties. Cubic zirconia, moissanite and glass are diamond simulants.","detail":"Every colorless stone sold as, or next to, a diamond is natural diamond, laboratory-grown diamond or a simulant. GIA names cubic zirconia as the most common diamond simulant today and moissanite as the next. A simulant gives itself away somewhere: in its optics, its weight or an instrument reading.","on_report":"GIA doesn't grade simulants; it identifies them on an Identification Report.","related":["moissanite","cubic-zirconia","identification-report","laboratory-grown-diamond","synthetic-gemstone"],"url":"https://digitalgemology.com/learn/glossary/simulant","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"Simulator cut grade","slug":"simulator-cut-grade","aka":["cut grade schedule"],"category":"How we measure","definition":"The simulator cut grade shows how a worse make of the same design looks. Each grade moves the crown angle, pavilion angle, girdle, table and culet by a fixed step.","detail":"It is a picture of a poorer make, not a measured light figure and not a laboratory grade. Poor, for example, lowers the crown angle 2.5° and raises the pavilion angle 5.2°. It also adds 3.5% to the girdle, 13 to the table and 3.5% to the culet.","how_we_measure":"The tool turns crown and pavilion facets about their girdle edges and rebuilds the table and culet. It rescales the stone to keep its weight. House cuts, the heart and the Platonic solids are shown as designed.","related":["3d-gemstone-simulator","cut-grade","crown-angle","pavilion-angle"],"url":"https://digitalgemology.com/learn/glossary/simulator-cut-grade","sources":["https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"Singly refractive","slug":"singly-refractive","aka":["isotropic","SR"],"category":"Optics","definition":"A singly refractive stone has one refractive index in every direction, so light stays one ray inside it. Diamond and cubic zirconia are singly refractive.","detail":"A singly refractive stone stays dark between crossed polarizers at every turn. A doubly refractive stone also stays dark when you look down its optic axis. So dark alone does not prove single refraction. Strain can make a singly refractive stone show patchy light.","how_we_measure":"The Polariscope shows diamond and cubic zirconia staying dark at every turn and tilt. Our build checks that no light passes them.","related":["doubly-refractive","polariscope","strain","crystal-system"],"url":"https://digitalgemology.com/learn/glossary/singly-refractive","sources":["https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/tools/polariscope","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Skull melting","slug":"skull-melting","aka":["skull crucible"],"category":"Treatments and origin","definition":"Skull melting is the method used to grow cubic zirconia. Radio-frequency heating melts the zirconia, and a cooled outer layer of unmelted powder, the 'skull', holds the melt.","detail":"Makers add a stabilizer, usually yttria or calcium oxide, so the crystal stays cubic. Other added elements give colored crystals.","related":["cubic-zirconia","synthetic-gemstone","simulant"],"url":"https://digitalgemology.com/learn/glossary/skull-melting","sources":[]},{"term":"Slightly Included (SI1, SI2)","slug":"si","aka":["SI","SI1","SI2"],"category":"Four Cs","definition":"SI1 and SI2 are the clarity grades for inclusions that are noticeable at 10x magnification.","detail":"SI inclusions are easy to find at 10x, and sometimes you can see them by eye. Many SI stones still look clean at arm's length, so SI is a popular range. SI2 is the last grade before the included range.","on_report":"Written SI1 or SI2 in the clarity grade line.","related":["vs","included","clarity-grade","clarity-plot"],"url":"https://digitalgemology.com/learn/glossary/si","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Snell's law","slug":"snells-law","aka":["law of refraction","refraction"],"category":"Optics","definition":"Snell's law sets the direction of refracted light: n₁ sin θ₁ = n₂ sin θ₂. The angles are measured from the normal to the surface.","detail":"Light bends toward the normal when it enters a material of higher index, and away from it when it leaves. When no angle satisfies the law, the light cannot leave and reflects totally.","how_we_measure":"Our light tracers refract rays by Snell's law. In doubly refractive stones, the simulator solves it for each of the two waves.","related":["refractive-index","critical-angle","total-internal-reflection","ray-tracing"],"url":"https://digitalgemology.com/learn/glossary/snells-law","sources":["https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/simulator"]},{"term":"Specific gravity","slug":"specific-gravity","aka":["SG","density"],"category":"Gemstone properties","definition":"Specific gravity compares a stone's weight with the weight of the same volume of water. It decides how much a stone of a given size weighs.","detail":"Cubic zirconia is 5.6 to 6.0, ruby and sapphire 4.00, diamond 3.52, moissanite 3.22, emerald 2.72 and amethyst 2.65. At the same size, a ruby weighs about 1.14 times as much as a diamond. A moissanite weighs about 0.91 times as much. So carat weight alone does not tell you the size.","how_we_measure":"Every weight on our cut pages comes from the model's measured volume and the stone's specific gravity. For cubic zirconia we use 5.8.","related":["weight-from-volume","face-up-size","heavy-liquid","carat-weight","gem-species"],"url":"https://digitalgemology.com/learn/glossary/specific-gravity","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-carat-size","https://digitalgemology.com/gemstones/ruby"]},{"term":"Spectroscope","slug":"spectroscope","aka":["hand spectroscope"],"category":"Gem testing","definition":"A spectroscope is a gem-testing instrument that spreads light from a stone into a spectrum. Dark lines and bands show where the stone absorbs.","detail":"A prism or a diffraction grating does the spreading. A grating spreads wavelengths evenly. A prism squeezes the red end and stretches the violet end. The lines often name the coloring element, not the stone.","how_we_measure":"Our Spectroscope computes the spectrum from measured spectra on a 1 nm grid, through 1 to 10 mm. You choose lamp light (illuminant A) or daylight (D65). Its prism is a 60° prism of N-SF11 flint glass. It is a teaching model, not a test of a real stone.","related":["absorption-spectrum","absorption-line","dispersion","cie-colorimetry"],"url":"https://digitalgemology.com/learn/glossary/spectroscope","sources":["https://digitalgemology.com/tools/spectroscope","https://digitalgemology.com/about/how-we-measure/spectroscope"]},{"term":"Spread (light performance)","slug":"spread-score","aka":["face-up size per carat (estimate)"],"category":"Light performance","definition":"Spread is an estimate of a cut's face-up size for its weight, compared with a stone 61.5% deep. Above 100 means the cut faces up larger for its weight than that reference.","detail":"Weight goes with the cube of the diameter, so the formula uses the cube root of the depth ratio. Spread estimates from depth alone. The face-up size on our gemstone cut pages is measured from each model's outline instead.","how_we_measure":"Spread = min(130, 100 × (61.5 ÷ depth)^(1/3)), with depth as a percentage of the width. It is the same in every gemstone. The Cut Performance Wheel draws values above 100 on its 100 ring and lists the true figure.","related":["face-up-size","overall-score","cut-performance-wheel","total-depth","carat-weight"],"url":"https://digitalgemology.com/learn/glossary/spread-score","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"Stability","slug":"stability","aka":["color stability","chemical stability"],"category":"Gemstone properties","definition":"Stability is a gem's resistance to damage from heat, light and chemicals.","detail":"Corundum is stable in normal wear, but boric acid powder can etch it. Sudden temperature changes can crack amethyst, and strong light can fade it. Heat can extend fractures in emerald and alter its fillers. Filled and dyed stones need more care than untreated or heated ones.","on_report":"Labs may add a care note, for example that a filler is unstable to heat and chemicals.","related":["durability","lead-glass-filling","emerald-oiling","irradiation","amethyst"],"url":"https://digitalgemology.com/learn/glossary/stability","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care"]},{"term":"Star facet","slug":"star-facet","aka":["stars","star facets"],"category":"Anatomy and proportions","definition":"Star facets are the 8 small triangular facets of a round brilliant's crown that touch the table.","detail":"Their reach toward the girdle is the star length. Long stars make the bezels smaller.","on_report":"Described on the proportion diagram by the star length.","related":["star-length","bezel-facet","table","crown"],"url":"https://digitalgemology.com/learn/glossary/star-facet","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length"]},{"term":"Star length","slug":"star-length","category":"Anatomy and proportions","definition":"Star length is how far the star facets reach from the table edge toward the girdle. It is a percentage of that distance.","detail":"GIA's Excellent range is 45 to 65% and its Very Good range 40 to 70%. On their own, star and lower half lengths can never take a stone below Good. The T73's stars are at 50%.","on_report":"On the proportion diagram; GIA reports it to the nearest 5%.","related":["star-facet","lower-half-length","bezel-facet","cut-grade"],"url":"https://digitalgemology.com/learn/glossary/star-length","sources":["https://digitalgemology.com/learn/diamond-anatomy/star-length-and-lower-half-length","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions"]},{"term":"Step cut","slug":"step-cut","aka":["step facets"],"category":"Cuts and shapes","definition":"A step cut is a cutting style with long facets in parallel rows, like steps, around a large table. The emerald cut and the Asscher are step cuts.","detail":"Step facets show the inside of a stone plainly, so color and clarity are easy to see. The cropped corners of the emerald cut protect a brittle stone.","on_report":"Named in the shape and cutting style line.","related":["brilliant-cut","emerald-cut","asscher-cut","shape-and-cutting-style"],"url":"https://digitalgemology.com/learn/glossary/step-cut","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-identity-section","https://digitalgemology.com/cuts/emerald-cut","https://digitalgemology.com/cuts/asscher"]},{"term":"STEP file","slug":"step-file","aka":["STEP",".stp"],"category":"Cutting and files","definition":"STEP is a standard CAD format that stores an exact solid, with each facet as a true plane. CAD programs such as FreeCAD, Rhino and Fusion open it to measure a stone and design a setting.","detail":"STEP keeps exact geometry, while mesh formats approximate it with triangles. A turned girdle can be one cylinder in a STEP solid.","how_we_measure":"Our facet counts come from the planar faces of each STEP solid. We check every STEP file with the Open CASCADE kernel. Our tools read a STEP cylinder as a ring of flat facets with the same cross-section area.","related":["3d-model","mesh","facet-count","turned-girdle"],"url":"https://digitalgemology.com/learn/glossary/step-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"STL file","slug":"stl-file","aka":["STL"],"category":"Cutting and files","definition":"STL is a triangle-mesh format and the standard for 3D printing. Slicers such as PrusaSlicer, Cura and Chitubox read it.","detail":"STL stores only the surface triangles, with no units. Jewelers print a stone in resin to test a setting before they cast metal.","how_we_measure":"Our STL files are watertight, so they print and cast without repair. Our tools also read your own STL files.","related":["mesh","3d-model","watertight"],"url":"https://digitalgemology.com/learn/glossary/stl-file","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/cuts/t73-round-brilliant","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Strain","slug":"strain","aka":["anomalous double refraction","ADR"],"category":"Gem testing","definition":"Strain is internal stress that makes a singly refractive stone weakly doubly refractive in patches. Between crossed polarizers it shows as patchy light that never goes dark all at once.","detail":"Many diamonds carry strain from growth. A truly doubly refractive stone goes dark everywhere at once, four times per turn. So patchy light points to strain. In GIA's study, moissanite showed no strain.","how_we_measure":"Our Polariscope offers an illustrative strain for diamond only, with path differences up to 140 nm. The direction of the slower ray wanders through every angle. The pattern is invented to show the effect, not measured from any stone.","related":["polariscope","blink-polariscope","singly-refractive","doubly-refractive"],"url":"https://digitalgemology.com/learn/glossary/strain","sources":["https://digitalgemology.com/about/how-we-measure/polariscope","https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Surface graining","slug":"surface-graining","category":"Clarity characteristics","definition":"Surface graining is lines on the surface of a diamond, caused by irregularities in its crystal structure. It is like internal graining, but on the surface.","detail":"It is a blemish. It comes from the crystal itself, not from damage or from polishing.","on_report":"A blemish; it is not always plotted.","related":["internal-graining","polish-lines","blemish"],"url":"https://digitalgemology.com/learn/glossary/surface-graining","sources":[]},{"term":"Symmetry","slug":"symmetry","aka":["symmetry grade"],"category":"Grading report","definition":"Symmetry is the report grade, from Excellent to Poor, for the exactness of a stone's outline. It also covers the shape, placement and alignment of the facets.","detail":"GIA sorts the faults into two kinds. Proportion faults include an off-center table or culet; facet faults include extra or misshapen facets. On a GIA round, Good symmetry limits the cut grade to Very Good, Fair to Good and Poor to Poor. Our Hearts & Arrows Scope and Cut Performance Wheel have their own symmetry measure, not the report grade.","on_report":"In the additional grading section of GIA and IGI reports. IGI gives a symmetry grade for every shape and color.","related":["polish","cut-grade","hearts-and-arrows","triple-excellent","extra-facet"],"url":"https://digitalgemology.com/learn/glossary/symmetry","sources":["https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence","https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined","https://digitalgemology.com/learn/historic-cuts/hearts-and-arrows-diamonds"]},{"term":"Symmetry (light performance)","slug":"symmetry-light-performance","aka":["pavilion symmetry score"],"category":"Light performance","definition":"In our light-performance scores, symmetry measures how evenly the pavilion facets of each angle family are cut. It reads 100 when every facet in a family has the same angle and distance from the center.","detail":"It is not the symmetry grade on a grading report. A report grade judges how exactly a real stone's facets are shaped, placed and aligned. Our score reads the cut's geometry only: a multi-angle pavilion counts as design, not as asymmetry. The Hearts & Arrows Scope gives a third, separate symmetry figure.","how_we_measure":"We group the pavilion facets into families. A new family starts where consecutive angles differ by more than 0.45°. For each family, we take the standard deviation of its angles, in degrees. We add 57.3 × the relative standard deviation of the facets' distances from the center. σ is the size-weighted mean of that sum, and Y = max(0, 100 − 6σ). It is the same in every gemstone.","related":["symmetry","hearts-and-arrows-symmetry","overall-score","cut-performance-wheel","mirror-image-symmetry"],"url":"https://digitalgemology.com/learn/glossary/symmetry-light-performance","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index"]},{"term":"Synthetic gemstone","slug":"synthetic-gemstone","aka":["laboratory-grown","lab-created","laboratory-created","lab-grown gemstone"],"category":"Treatments and origin","definition":"A synthetic gemstone is made in a laboratory and has essentially the same chemistry and properties as the natural gem. A synthetic ruby is ruby, made by people.","detail":"US FTC Guides require a word such as 'laboratory-grown' or 'laboratory-created' right before the gem name, equally visible. Moissanite in jewelry is synthetic moissanite. Sold as an imitation of diamond, it is a diamond simulant.","on_report":"Labs state laboratory-grown material as such.","related":["simulant","flame-fusion","flux-growth","hydrothermal-growth","moissanite","disclosure"],"url":"https://digitalgemology.com/learn/glossary/synthetic-gemstone","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-certificate-vs-diamond-report"]},{"term":"T73 benchmark","slug":"t73-benchmark","aka":["T73 Round Brilliant","benchmark round"],"category":"How we measure","definition":"The T73 benchmark is the round brilliant that our tools compare every cut with, in the same gemstone. It has Tolkowsky's 1919 angles and table, and 73 facets: 57 plus 16 girdle facets.","detail":"Its main crown angle is 34.5° and its main pavilion angle 40.75°, with a 53% table. In diamond it reads 93 for brilliance, 78 for fire and 80 for scintillation in our census. It is a reference, not a top score: five cuts score higher overall in diamond.","how_we_measure":"The Cut Performance Wheel draws the T73 as a dashed outline. Each cut-page bar shows a thin mark for it. The Cut Grading Tool traces its brilliance and light maps on the T73.","related":["cut-performance-wheel","cut-performance-index","brilliance","crown-angle","pavilion-angle"],"url":"https://digitalgemology.com/learn/glossary/t73-benchmark","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/cuts/t73-round-brilliant","https://digitalgemology.com/about/how-we-measure/cut-grading-tool"]},{"term":"Table","slug":"table","aka":["table facet"],"category":"Anatomy and proportions","definition":"The table is the large flat facet on top of a cut stone, an octagon on a round brilliant. Much of the light enters through it.","detail":"Its width as a percentage of the diameter is the table size. Face up, you look through it straight down to the pavilion and the culet.","on_report":"Its width appears as \"table\" on the proportion diagram.","related":["table-size","crown","star-facet","culet"],"url":"https://digitalgemology.com/learn/glossary/table","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/diamond-anatomy/diamond-table-percentage"]},{"term":"Table size","slug":"table-size","aka":["table percentage","table percent"],"category":"Anatomy and proportions","definition":"Table size is the width of the table as a percentage of the average diameter.","detail":"GIA's Excellent limit for the table alone is 52 to 62%. GIA also grades it together with the crown and pavilion angles. At the same crown angle, a bigger table makes a lower crown and a shallower stone. Bigger is not automatically brighter.","on_report":"On the proportion diagram as \"table\". IGI's Ideal band is 53 to 60%.","related":["table","crown-height","crown-angle","cut-grade","ideal-cut"],"url":"https://digitalgemology.com/learn/glossary/table-size","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-table-percentage","https://digitalgemology.com/learn/grading-reports/diamond-proportion-diagram","https://digitalgemology.com/learn/grading-reports/what-ideal-cut-means"]},{"term":"Thermal diamond tester","slug":"thermal-diamond-tester","aka":["thermal tester","diamond tester","heat tester"],"category":"Gem testing","definition":"A thermal diamond tester measures how fast a stone draws heat from a hot tip. Diamond conducts heat very well, so it reads “diamond”.","detail":"Moissanite conducts heat almost as well. In GIA's 1997 study, every moissanite read “diamond” on every thermal tester tried. So a thermal “diamond” reading means diamond or moissanite.","related":["electrical-conductivity-tester","screening-device","polariscope"],"url":"https://digitalgemology.com/learn/glossary/thermal-diamond-tester","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Three-phase inclusion","slug":"three-phase-inclusion","aka":["multiphase inclusion"],"category":"Clarity characteristics","definition":"A three-phase inclusion is a small cavity that holds a liquid, a gas bubble and a crystal. The crystal is often a cube of salt (halite).","detail":"It is a classic feature of Colombian emerald. Similar inclusions occur in emeralds from other places, such as Zambia. So they suggest an origin but do not prove it.","related":["emerald","jardin","origin-report","inclusion"],"url":"https://digitalgemology.com/learn/glossary/three-phase-inclusion","sources":[]},{"term":"Tier","slug":"tier","aka":["facet tier"],"category":"Cutting and files","definition":"A tier is a set of facets cut at the same angle, at several index positions around the stone. A faceting design lists its tiers in cutting order.","detail":"Round brilliants have tiers such as the pavilion mains, lower halves, crown mains, stars and upper halves. The T73 has 3 crown tiers and 2 pavilion tiers.","how_we_measure":"Our tier table lists the angle, indices and cutting order of every tier. Our cut pages take the main crown and pavilion angles from the tiers that the design gives.","related":["facet","index-gear","tier-table","cutting-order","gemcad-asc"],"url":"https://digitalgemology.com/learn/glossary/tier","sources":["https://digitalgemology.com/about/files","https://digitalgemology.com/cuts/t73-round-brilliant","https://digitalgemology.com/about/how-we-measure/3d-models"]},{"term":"Tier table","slug":"tier-table","aka":[".csv tier table"],"category":"Cutting and files","definition":"A tier table lists the angle, index positions and cutting order of every tier in a faceting design. Ours comes as a .csv file that opens in any spreadsheet.","detail":"It lists every step in the order you cut it. It opens in Excel, Google Sheets, LibreOffice Calc and Numbers.","how_we_measure":"Each set of our cutting files includes a tier table.","related":["tier","cutting-order","cutting-file"],"url":"https://digitalgemology.com/learn/glossary/tier-table","sources":["https://digitalgemology.com/about/files"]},{"term":"Tilt (light performance)","slug":"tilt-light-performance","aka":["tilt retention","tilt score"],"category":"Light performance","definition":"Tilt is how much of its face-up brilliance a cut keeps when the stone tilts. Our tilt score compares brilliance at a 20° tilt with brilliance face-up, capped at 100.","detail":"A stone on a hand is seldom seen exactly face-up. A cut that holds its light when tipped looks steadier. The cap at 100 stops a dim cut from scoring well only because it has little light to lose. The T73 round brilliant in diamond scores 92.","how_we_measure":"The census runs again with the stone tilted 20°, with 2,200 rays. T = min(100, 100 × B(20°) ÷ B(0°)). The Brightness & Tilt Analysis gives a fuller picture: its figures at 13 tilts from −30° to 30°.","related":["brilliance","light-census","brightness-and-tilt-analysis","overall-score","head-shadow"],"url":"https://digitalgemology.com/learn/glossary/tilt-light-performance","sources":["https://digitalgemology.com/about/how-we-measure/performance-wheel","https://digitalgemology.com/learn/cut-performance-index","https://digitalgemology.com/about/how-we-measure/brightness-tilt"]},{"term":"Tolkowsky ideal cut","slug":"tolkowsky-ideal-cut","aka":["Tolkowsky cut","Tolkowsky brilliant","1919 ideal cut"],"category":"Cuts and shapes","definition":"The Tolkowsky ideal cut is the round brilliant that Marcel Tolkowsky calculated in his 1919 book Diamond Design. It has a 40.75° (40° 45′) pavilion, a 34.5° (34° 30′) crown and a 53% table.","detail":"He worked from diamond's refractive index, 2.417, and its critical angle, 24° 26′. His model was a 2D slice with light from straight above and at most two reflections. Our T57 is his plan with a knife-edge girdle, and our T73 adds 16 girdle facets.","related":["round-brilliant","pavilion-angle","crown-angle","knife-edge-girdle","ideal-cut","critical-angle"],"url":"https://digitalgemology.com/learn/glossary/tolkowsky-ideal-cut","sources":["https://digitalgemology.com/learn/historic-cuts/tolkowsky-ideal-cut-1919","https://digitalgemology.com/learn/diamond-anatomy/diamond-crown-angle","https://digitalgemology.com/learn/diamond-anatomy/diamond-pavilion-angle","https://digitalgemology.com/cuts/t57-round-brilliant"]},{"term":"Tone","slug":"tone","aka":["color tone"],"category":"Colored gemstones","definition":"Tone is how light or dark a gem's color is, from very light to very dark.","detail":"The most valued blue sapphires are medium to medium-dark in tone. Stones that are too dark or too light are worth less. A ruby that is too light may be called pink sapphire.","related":["hue","saturation","pink-sapphire","sapphire"],"url":"https://digitalgemology.com/learn/glossary/tone","sources":["https://digitalgemology.com/learn/four-cs/fancy-color-diamonds-and-fluorescence"]},{"term":"Total depth","slug":"total-depth","aka":["depth percentage","depth %"],"category":"Anatomy and proportions","definition":"Total depth is the height of a stone from table to culet as a percentage of its average diameter. It equals crown height + girdle thickness + pavilion depth.","detail":"To check it, divide the depth in the measurements by the average diameter. GIA's Excellent range is 57.5 to 63%, and over 63% usually means extra weight below the face. For a fancy shape, depth is a percentage of the width.","on_report":"On the proportion diagram as \"total depth\". IGI's Ideal band is 58.5 to 62.5%.","related":["measurements","crown-height","pavilion-depth","girdle-thickness","weight-ratio"],"url":"https://digitalgemology.com/learn/glossary/total-depth","sources":["https://digitalgemology.com/learn/diamond-anatomy/diamond-measurements-explained","https://digitalgemology.com/learn/diamond-anatomy/how-to-read-diamond-proportions","https://digitalgemology.com/learn/grading-reports/how-to-read-a-diamond-grading-report","https://digitalgemology.com/learn/bow-tie/bow-tie-and-the-grading-report"]},{"term":"Total internal reflection","slug":"total-internal-reflection","aka":["TIR"],"category":"Optics","definition":"Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the light reflects back into the stone.","detail":"It is how a pavilion sends light back up through the crown. Total internal reflection loses no light. Facets cut too shallow for the critical angle let light out instead.","how_we_measure":"All four of our light measures treat total internal reflection as lossless. Only the simulator also counts partial reflections at facets where light can escape.","related":["critical-angle","refractive-index","fresnel-reflection","leakage"],"url":"https://digitalgemology.com/learn/glossary/total-internal-reflection","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/about/how-we-measure/light-measures","https://digitalgemology.com/about/how-we-measure/limits"]},{"term":"Toughness","slug":"toughness","aka":["brittleness"],"category":"Gemstone properties","definition":"Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.","detail":"Corundum has excellent toughness, amethyst good, and emerald fair to good. Diamond is good along its cleavage directions and exceptional in others. Moissanite showed no cleavage in GIA's tests, but its girdle can chip under hard wear. Fractures and inclusions can lower the toughness of a single stone.","related":["hardness","cleavage","fracture","durability"],"url":"https://digitalgemology.com/learn/glossary/toughness","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-properties","https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-durability-and-care","https://digitalgemology.com/gemstones/emerald"]},{"term":"Trade color term","slug":"trade-color-term","aka":["trade name","color trade name"],"category":"Colored gemstones","definition":"A trade color term is a market name for a fine color, such as pigeon's blood, royal blue or cornflower. It is not a gemological standard.","detail":"No universally accepted definitions exist for these terms. Each lab that uses one sets its own criteria. So the same stone can get the term from one lab and not from another. Read which lab issued the term, and on what criteria.","on_report":"Some labs add it as a separate color statement or appendix, not as the variety name.","related":["pigeons-blood","royal-blue","kashmir-sapphire","padparadscha","hue"],"url":"https://digitalgemology.com/learn/glossary/trade-color-term","sources":[]},{"term":"Trapiche","slug":"trapiche","aka":["trapiche emerald","trapiche ruby"],"category":"Gemstone properties","definition":"Trapiche is a pattern of six arms of inclusions that radiate from the center of a crystal. You see it in a slice cut across the crystal's main axis.","detail":"The name is the Spanish word for the cogwheels of a sugar mill. It is best known in Colombian emerald and also occurs in ruby from Mong Hsu, Myanmar. The arms are part of the crystal's growth, not reflected light. So, unlike a star, the pattern does not move when the stone or the light moves.","related":["emerald","ruby","asterism","inclusion"],"url":"https://digitalgemology.com/learn/glossary/trapiche","sources":[]},{"term":"Treatment","slug":"treatment","aka":["enhancement","gem treatment","diamond treatment","treated diamond"],"category":"Treatments and origin","definition":"A treatment is any process, other than cutting and polishing, that changes a gem's color or apparent clarity. Anyone who sells a treated gem must disclose the treatment.","detail":"Common treatments in colored stones are heating, diffusion, filling, irradiation, dyeing and coating. Some are stable and permanent; others need special care. For diamonds, GIA grades permanent treatments, such as HPHT color treatment and laser drilling. Unstable ones, such as coating and fracture filling, get only an Identification Report.","on_report":"Labs list detected treatments under Comments or Further information. On a GIA diamond report, a color treatment also adds an asterisk to the color grade.","related":["heat-treatment","beryllium-diffusion","clarity-enhancement","color-treatment","laser-drilling","disclosure"],"url":"https://digitalgemology.com/learn/glossary/treatment","sources":["https://digitalgemology.com/learn/grading-reports/diamond-report-comments-treatments-origin"]},{"term":"Triple Excellent","slug":"triple-excellent","aka":["3EX","Triple EX"],"category":"Grading report","definition":"Triple Excellent is trade shorthand for a GIA round brilliant with Excellent cut, Excellent polish and Excellent symmetry.","detail":"It is the entry ticket for the AGS Ideal Report by GIA, but it doesn't guarantee that a stone qualifies. GIA's cut grade itself allows Excellent with Very Good polish or symmetry.","on_report":"Not a line of its own: read it from the cut, polish and symmetry lines.","related":["cut-grade","polish","symmetry","ags-ideal-report"],"url":"https://digitalgemology.com/learn/glossary/triple-excellent","sources":["https://digitalgemology.com/learn/grading-reports/what-ideal-cut-means","https://digitalgemology.com/learn/grading-reports/polish-symmetry-and-fluorescence"]},{"term":"Turned girdle","slug":"turned-girdle","aka":["round girdle","cylindrical girdle"],"category":"Cutting and files","definition":"A turned girdle is a girdle with a smooth round edge, shaped as a cylinder, instead of flat girdle facets. CAD often gives a round stone this kind of girdle.","detail":"A turned girdle is one surface in a solid but many facets on a faceting machine. So a cut page can give two facet counts.","how_we_measure":"Our tools read a STEP cylinder as a ring of flat facets with the same cross-section area.","related":["facet-count","step-file","girdle"],"url":"https://digitalgemology.com/learn/glossary/turned-girdle","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Twinning wisp","slug":"twinning-wisp","aka":["twinning wisps"],"category":"Clarity characteristics","definition":"A twinning wisp is a series of pinpoints, clouds or crystals that forms in a diamond's growth plane. It comes with crystal distortion and twinning planes.","detail":"It forms along a twinning plane, where the crystal structure changes direction. GIA lists it among the inclusions.","on_report":"Drawn in red on the clarity plot of a full GIA report and named in the key to symbols. A Dossier has no plot but can name it in its list of up to four.","related":["cloud","pinpoint","internal-graining","inclusion"],"url":"https://digitalgemology.com/learn/glossary/twinning-wisp","sources":[]},{"term":"Type IIb diamond","slug":"type-iib-diamond","aka":["type IIb","boron-blue diamond"],"category":"Diamond and simulants","definition":"A type IIb diamond contains boron, which can make it blue or gray. The boron also lets it conduct electricity.","detail":"That is why a blue diamond can read \"moissanite\" on an electrical tester. Labs grow boron-blue diamonds too, so a blue stone that conducts needs a gem lab.","related":["electrical-conductivity-tester","fancy-color-diamond","laboratory-grown-diamond"],"url":"https://digitalgemology.com/learn/glossary/type-iib-diamond","sources":["https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite"]},{"term":"Uniaxial","slug":"uniaxial","category":"Optics","definition":"A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.","detail":"In a uniaxial stone, the ordinary ray has one index in every direction. The extraordinary ray's index changes with direction. Moissanite and emerald belong to the hexagonal crystal system; ruby, sapphire and amethyst to the trigonal.","how_we_measure":"The Polariscope's conoscope shows a uniaxial interference figure, and its red plate reads the optic sign.","related":["biaxial","optic-axis","optic-sign","interference-figure","crystal-system"],"url":"https://digitalgemology.com/learn/glossary/uniaxial","sources":["https://digitalgemology.com/about/how-we-measure/refractometer","https://digitalgemology.com/about/how-we-measure/polariscope"]},{"term":"Upper half facet","slug":"upper-half-facet","aka":["upper halves","upper girdle facets","upper girdle halves"],"category":"Anatomy and proportions","definition":"Upper half facets are the 16 small triangles along the crown edge of a round brilliant. There are two between each pair of bezels.","detail":"With the 8 stars and the 8 bezels, they make a 32-facet crown. The Peruzzi cut introduced that crown in the late 1600s.","related":["bezel-facet","star-facet","painting-and-digging","crown"],"url":"https://digitalgemology.com/learn/glossary/upper-half-facet","sources":["https://digitalgemology.com/learn/diamond-anatomy/parts-of-a-diamond","https://digitalgemology.com/learn/historic-cuts/peruzzi-cut-diamond"]},{"term":"Very light (N–R)","slug":"very-light-color","aka":["N-R"],"category":"Color and fluorescence","definition":"Very light is the D-to-Z range from N to R: a yellow or brown tint that is easy to see.","detail":"It is the fourth of the five D-to-Z ranges. Past R, the Light range runs from S to Z.","related":["faint-color","light-color","color-grade"],"url":"https://digitalgemology.com/learn/glossary/very-light-color","sources":["https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z"]},{"term":"Very Slightly Included (VS1, VS2)","slug":"vs","aka":["VS","VS1","VS2"],"category":"Four Cs","definition":"VS1 and VS2 are the clarity grades for small inclusions that a grader finds with some effort at 10x magnification.","detail":"VS inclusions are minor, and you rarely see them with the unaided eye. The step from VVS2 to VS1 depends on the type and size of the characteristics, not only their number.","on_report":"Written VS1 or VS2 in the clarity grade line.","related":["vvs","si","clarity-grade","10x-magnification"],"url":"https://digitalgemology.com/learn/glossary/vs","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale"]},{"term":"Very, Very Slightly Included (VVS1, VVS2)","slug":"vvs","aka":["VVS","VVS1","VVS2"],"category":"Four Cs","definition":"VVS1 and VVS2 are the clarity grades for very small inclusions. Even a skilled grader has difficulty finding them at 10x.","detail":"VVS1 is the better of the two. At VVS1, our simulator plots a single pinpoint.","on_report":"Written VVS1 or VVS2 in the clarity grade line.","related":["internally-flawless","vs","pinpoint","clarity-grade"],"url":"https://digitalgemology.com/learn/glossary/vvs","sources":["https://digitalgemology.com/learn/four-cs/diamond-clarity-scale","https://digitalgemology.com/learn/grading-reports/diamond-clarity-plot"]},{"term":"Watertight","slug":"watertight","aka":["closed mesh"],"category":"Cutting and files","definition":"A watertight model is a closed surface with no holes, so it encloses a solid. In a closed mesh, exactly two triangles share every edge.","detail":"Only a watertight model has a volume, a weight and a clear inside for light. Many display models are not closed.","how_we_measure":"Each build stops if one of our meshes is not closed. A file of your own must enclose the same solid as its facets, to within 0.5% of its volume.","related":["mesh","3d-model","convex","measure-your-own-stone"],"url":"https://digitalgemology.com/learn/glossary/watertight","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/about/how-we-measure/your-own-file"]},{"term":"Weight from volume","slug":"weight-from-volume","aka":["carat weight formula"],"category":"How we measure","definition":"Weight from volume is how we calculate a model's weight: m = V × ρ ÷ 200 carats. V is the volume in mm³ and ρ the specific gravity in g/cm³.","detail":"The 200 comes from 1 carat = 0.2 g and 1 cm³ = 1,000 mm³. At a given shape, weight scales with the cube of size, so twice the size weighs eight times as much. At its reference size, the T73 weighs 1.99 ct in diamond and 3.28 ct in cubic zirconia.","how_we_measure":"Each build measures every model's volume from its mesh, by the divergence theorem over its triangles. The simulator scales the model by the cube root of the weight ratio to reach the carat weight you choose.","related":["specific-gravity","reference-size","carat-weight","mesh"],"url":"https://digitalgemology.com/learn/glossary/weight-from-volume","sources":["https://digitalgemology.com/about/how-we-measure/3d-models","https://digitalgemology.com/cuts/t73-round-brilliant"]},{"term":"Weight ratio","slug":"weight-ratio","category":"Four Cs","definition":"Weight ratio is the part of GIA's cut grade that compares a diamond's weight with its diameter. A stone can carry too much or too little weight for its width.","detail":"A thick girdle and a deep stone add weight that doesn't show face up. In our simulator, a Poor-cut 1.00 ct T73 is only 6.16 mm across. That is the width of an Excellent T73 of about 0.86 ct. GIA uses the total depth to help judge weight ratio, and its Excellent limit is 63%.","on_report":"Not printed as its own line: read it from the total depth and girdle on the proportion diagram.","related":["cut-grade","total-depth","girdle-thickness","durability-cut-grade"],"url":"https://digitalgemology.com/learn/glossary/weight-ratio","sources":["https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined","https://digitalgemology.com/learn/four-cs/diamond-cut-grade-excellent-vs-poor","https://digitalgemology.com/learn/diamond-anatomy/diamond-girdle-thickness"]},{"term":"Windowing","slug":"windowing","aka":["window","see-through area"],"category":"Light performance","definition":"A window is a face-up area through which you can see straight through a stone. It forms where pavilion facets are too shallow to reflect light, so light passes out of the back.","detail":"Windowing depends on the angle and the refractive index together. A pavilion that works in diamond can window in amethyst. Amethyst's critical angle (40.4°) is much larger than diamond's (24.4°).","how_we_measure":"On cut pages, the head-on test reads “Minor windowing” when 90% to under 99% of the pavilion reflects head-on light. The light map shows windows as leakage.","related":["head-on-light","critical-angle","leakage","light-map","guide-minimum-pavilion-angle"],"url":"https://digitalgemology.com/learn/glossary/windowing","sources":["https://digitalgemology.com/about/how-we-measure/gemstone-constants","https://digitalgemology.com/cuts/t73-round-brilliant","https://digitalgemology.com/gemstones/amethyst"]}]}