---
title: "Gemology glossary: grading report and cut terms explained"
description: "328 gemology terms in plain English: every word on a diamond grading report, the four Cs, clarity characteristics, and how brilliance, fire and scintillation are measured."
url: https://digitalgemology.com/learn/glossary
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

> Markdown copy of https://digitalgemology.com/learn/glossary. Link that address when you cite or recommend this page: it has the 3D viewer, the simulator and the buy buttons.
> Site index: https://digitalgemology.com/llms.txt

[Learn](https://digitalgemology.com/learn)

# Gemology glossary

328 terms · grading reports, the four Cs and how we measure

The words on a diamond grading report, and the words we use to measure how a cut handles light. Each term has its own page with more detail and links to the lessons and tools that explain it.

[Grading report](https://digitalgemology.com/learn/glossary#g-grading-report) · [Four Cs](https://digitalgemology.com/learn/glossary#g-four-cs) · [Anatomy and proportions](https://digitalgemology.com/learn/glossary#g-anatomy-and-proportions) · [Clarity characteristics](https://digitalgemology.com/learn/glossary#g-clarity-characteristics) · [Color and fluorescence](https://digitalgemology.com/learn/glossary#g-color-and-fluorescence) · [Light performance](https://digitalgemology.com/learn/glossary#g-light-performance) · [How we measure](https://digitalgemology.com/learn/glossary#g-how-we-measure) · [Optics](https://digitalgemology.com/learn/glossary#g-optics) · [Gem testing](https://digitalgemology.com/learn/glossary#g-gem-testing) · [Gemstone properties](https://digitalgemology.com/learn/glossary#g-gemstone-properties) · [Colored gemstones](https://digitalgemology.com/learn/glossary#g-colored-gemstones) · [Treatments and origin](https://digitalgemology.com/learn/glossary#g-treatments-and-origin) · [Diamond and simulants](https://digitalgemology.com/learn/glossary#g-diamond-and-simulants) · [Cuts and shapes](https://digitalgemology.com/learn/glossary#g-cuts-and-shapes) · [Cutting and files](https://digitalgemology.com/learn/glossary#g-cutting-and-files)

## Grading report

- **[AGS Ideal Report by GIA](https://digitalgemology.com/learn/glossary/ags-ideal-report)**: 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.
- **[Appraisal](https://digitalgemology.com/learn/glossary/appraisal)**: An appraisal is a statement of what a stone or a piece of jewelry is worth. A diamond grading report is not an appraisal.
- **[As mounting permits](https://digitalgemology.com/learn/glossary/as-mounting-permits)**: "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.
- **[ASET map](https://digitalgemology.com/learn/glossary/aset-map)**: 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.
- **[Asterisk after the color grade](https://digitalgemology.com/learn/glossary/asterisk-color-grade)**: An asterisk (*) after a GIA color grade means that the lab detected a color treatment. Comments explains what it is.
- **[Clarity plot](https://digitalgemology.com/learn/glossary/clarity-plot)**: 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.
- **[Comments](https://digitalgemology.com/learn/glossary/comments)**: 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.
- **[Diamond Dossier](https://digitalgemology.com/learn/glossary/diamond-dossier)**: 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.
- **[Diamond grading report](https://digitalgemology.com/learn/glossary/diamond-grading-report)**: 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.
- **[Ideal cut](https://digitalgemology.com/learn/glossary/ideal-cut)**: "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".
- **[Identification Report](https://digitalgemology.com/learn/glossary/identification-report)**: 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.
- **[Inscription](https://digitalgemology.com/learn/glossary/inscription)**: 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.
- **[Key to symbols](https://digitalgemology.com/learn/glossary/key-to-symbols)**: The key to symbols is the list beside the clarity plot. It names the kinds of clarity characteristic on that plot.
- **[Laboratory-Grown Diamond Quality Assessment](https://digitalgemology.com/learn/glossary/laboratory-grown-diamond-quality-assessment)**: 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.
- **[Laboratory-Grown Diamond Report](https://digitalgemology.com/learn/glossary/laboratory-grown-diamond-report)**: 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.
- **[Light Performance Report (IGI)](https://digitalgemology.com/learn/glossary/igi-light-performance-report)**: 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.
- **[Measurements](https://digitalgemology.com/learn/glossary/measurements)**: 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.
- **[Polish](https://digitalgemology.com/learn/glossary/polish)**: 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.
- **[Proportion diagram](https://digitalgemology.com/learn/glossary/proportion-diagram)**: 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.
- **[Report Check](https://digitalgemology.com/learn/glossary/report-check)**: Report Check is GIA's online database. Enter a report number to see what GIA itself recorded; IGI's equivalent is Verify Your Report.
- **[Report date](https://digitalgemology.com/learn/glossary/report-date)**: The report date is the day the lab examined the stone. Everything on the report is true for that day.
- **[Report number](https://digitalgemology.com/learn/glossary/report-number)**: The report number is the unique number a lab gives a report. GIA registers it in its database, so anyone can look it up.
- **[Security features](https://digitalgemology.com/learn/glossary/security-features)**: 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.
- **[Shape and cutting style](https://digitalgemology.com/learn/glossary/shape-and-cutting-style)**: 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".
- **[Symmetry](https://digitalgemology.com/learn/glossary/symmetry)**: 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.
- **[Triple Excellent](https://digitalgemology.com/learn/glossary/triple-excellent)**: Triple Excellent is trade shorthand for a GIA round brilliant with Excellent cut, Excellent polish and Excellent symmetry.

## Four Cs

- **[10x magnification](https://digitalgemology.com/learn/glossary/10x-magnification)**: 10x magnification is the standard view for diamond clarity grading: ten times life size, with a loupe or a microscope.
- **[Carat weight](https://digitalgemology.com/learn/glossary/carat-weight)**: Carat weight is the weight of a gemstone. One carat is 200 milligrams, a fifth of a gram, and it has 100 points.
- **[Clarity grade](https://digitalgemology.com/learn/glossary/clarity-grade)**: 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).
- **[Color grade](https://digitalgemology.com/learn/glossary/color-grade)**: 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.
- **[Cut grade](https://digitalgemology.com/learn/glossary/cut-grade)**: 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.
- **[Durability (cut grade)](https://digitalgemology.com/learn/glossary/durability-cut-grade)**: 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.
- **[Flawless (FL)](https://digitalgemology.com/learn/glossary/flawless)**: Flawless (FL) is the top clarity grade: a skilled grader finds no inclusions and no blemishes at 10x magnification.
- **[Four Cs](https://digitalgemology.com/learn/glossary/four-cs)**: 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.
- **[Included (I1, I2, I3)](https://digitalgemology.com/learn/glossary/included)**: I1, I2 and I3 are the lowest clarity grades. Their inclusions are obvious at 10x and can affect transparency and brilliance.
- **[Internally Flawless (IF)](https://digitalgemology.com/learn/glossary/internally-flawless)**: Internally Flawless (IF) is the clarity grade for a diamond with no inclusions at 10x. Only blemishes on the surface show.
- **[Magic sizes](https://digitalgemology.com/learn/glossary/magic-sizes)**: Magic sizes are round-number weights, such as half a carat or one carat. They sell for more because of demand, not looks.
- **[Slightly Included (SI1, SI2)](https://digitalgemology.com/learn/glossary/si)**: SI1 and SI2 are the clarity grades for inclusions that are noticeable at 10x magnification.
- **[Very Slightly Included (VS1, VS2)](https://digitalgemology.com/learn/glossary/vs)**: VS1 and VS2 are the clarity grades for small inclusions that a grader finds with some effort at 10x magnification.
- **[Very, Very Slightly Included (VVS1, VVS2)](https://digitalgemology.com/learn/glossary/vvs)**: VVS1 and VVS2 are the clarity grades for very small inclusions. Even a skilled grader has difficulty finding them at 10x.
- **[Weight ratio](https://digitalgemology.com/learn/glossary/weight-ratio)**: 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.

## Anatomy and proportions

- **[Average diameter](https://digitalgemology.com/learn/glossary/average-diameter)**: The average diameter of a round is the mean of its minimum and maximum diameters, measured across the girdle.
- **[Belly](https://digitalgemology.com/learn/glossary/belly)**: The belly is the wide middle of an elongated stone such as an oval, pear or marquise.
- **[Bezel facet](https://digitalgemology.com/learn/glossary/bezel-facet)**: Bezel facets are the 8 large four-sided facets on the crown of a round brilliant. They run from the table to the girdle.
- **[Crown](https://digitalgemology.com/learn/glossary/crown)**: 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.
- **[Crown angle](https://digitalgemology.com/learn/glossary/crown-angle)**: The crown angle is the slope of the bezel facets, measured against the plane of the girdle.
- **[Crown height](https://digitalgemology.com/learn/glossary/crown-height)**: Crown height is the height of the crown, from the girdle to the table. It is a percentage of the average diameter.
- **[Culet](https://digitalgemology.com/learn/glossary/culet)**: 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.
- **[Culet size](https://digitalgemology.com/learn/glossary/culet-size)**: 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.
- **[Girdle](https://digitalgemology.com/learn/glossary/girdle)**: 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.
- **[Girdle facets](https://digitalgemology.com/learn/glossary/girdle-facets)**: Girdle facets are the small facets cut around the girdle itself, such as the 16 on the T73.
- **[Girdle thickness](https://digitalgemology.com/learn/glossary/girdle-thickness)**: 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".
- **[Head](https://digitalgemology.com/learn/glossary/head)**: 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.
- **[Keel](https://digitalgemology.com/learn/glossary/keel)**: 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.
- **[Knife-edge girdle](https://digitalgemology.com/learn/glossary/knife-edge-girdle)**: A knife-edge girdle has no girdle band at all: the crown and the pavilion meet in a sharp line.
- **[Length-to-width ratio](https://digitalgemology.com/learn/glossary/length-to-width-ratio)**: The length-to-width ratio of a fancy shape is its length divided by its width, both taken from the measurements.
- **[Lower half facet](https://digitalgemology.com/learn/glossary/lower-half-facet)**: 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.
- **[Lower half length](https://digitalgemology.com/learn/glossary/lower-half-length)**: Lower half length is how far the lower half facets reach from the girdle toward the culet. It is a percentage of that distance.
- **[Painting and digging](https://digitalgemology.com/learn/glossary/painting-and-digging)**: Painting and digging out are small tilts of the half facets along the girdle. Cutters use them to save weight.
- **[Pavilion](https://digitalgemology.com/learn/glossary/pavilion)**: 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.
- **[Pavilion angle](https://digitalgemology.com/learn/glossary/pavilion-angle)**: The pavilion angle is the slope of the pavilion mains, measured against the plane of the girdle.
- **[Pavilion depth](https://digitalgemology.com/learn/glossary/pavilion-depth)**: Pavilion depth is the depth of the pavilion, from the girdle to the culet. It is a percentage of the average diameter.
- **[Pavilion main](https://digitalgemology.com/learn/glossary/pavilion-main)**: Pavilion mains are the 8 long facets of a round brilliant's pavilion that run from the girdle to the culet.
- **[Proportions](https://digitalgemology.com/learn/glossary/proportions)**: 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.
- **[Shoulder](https://digitalgemology.com/learn/glossary/shoulder)**: The shoulder of an elongated stone is where the outline curves away from the end.
- **[Star facet](https://digitalgemology.com/learn/glossary/star-facet)**: Star facets are the 8 small triangular facets of a round brilliant's crown that touch the table.
- **[Star length](https://digitalgemology.com/learn/glossary/star-length)**: Star length is how far the star facets reach from the table edge toward the girdle. It is a percentage of that distance.
- **[Table](https://digitalgemology.com/learn/glossary/table)**: 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.
- **[Table size](https://digitalgemology.com/learn/glossary/table-size)**: Table size is the width of the table as a percentage of the average diameter.
- **[Total depth](https://digitalgemology.com/learn/glossary/total-depth)**: 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.
- **[Upper half facet](https://digitalgemology.com/learn/glossary/upper-half-facet)**: Upper half facets are the 16 small triangles along the crown edge of a round brilliant. There are two between each pair of bezels.

## Clarity characteristics

- **[Abrasion](https://digitalgemology.com/learn/glossary/abrasion)**: An abrasion is a row of tiny nicks along the facet junctions of a cut diamond. It makes the edges look white or fuzzy.
- **[Bearded girdle](https://digitalgemology.com/learn/glossary/bearded-girdle)**: A bearded girdle has very small feathers that run from the girdle surface into the stone. They can come from the cutting process.
- **[Blemish](https://digitalgemology.com/learn/glossary/blemish)**: A blemish is a clarity characteristic confined to the surface of a diamond.
- **[Bruise](https://digitalgemology.com/learn/glossary/bruise)**: A bruise is a tiny area of impact damage with very small, root-like feathers. It usually sits at a facet junction.
- **[Burn mark](https://digitalgemology.com/learn/glossary/burn-mark)**: 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.
- **[Cavity](https://digitalgemology.com/learn/glossary/cavity)**: 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.
- **[Chip](https://digitalgemology.com/learn/glossary/chip)**: 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.
- **[Clarity characteristic](https://digitalgemology.com/learn/glossary/clarity-characteristic)**: A clarity characteristic is any inclusion or blemish in or on a diamond. Together they set the clarity grade.
- **[Cloud](https://digitalgemology.com/learn/glossary/cloud)**: A cloud is many tightly grouped pinpoints that are too small to see one by one but together look hazy.
- **[Crystal](https://digitalgemology.com/learn/glossary/crystal-inclusion)**: A crystal is a mineral crystal contained in a diamond.
- **[Curved striae](https://digitalgemology.com/learn/glossary/curved-striae)**: Curved striae are curved growth lines or color bands in a flame-fusion synthetic. Natural corundum grows with straight, angular zoning instead.
- **[Etch channel](https://digitalgemology.com/learn/glossary/etch-channel)**: 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.
- **[Extra facet](https://digitalgemology.com/learn/glossary/extra-facet)**: 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.
- **[Feather](https://digitalgemology.com/learn/glossary/feather)**: A feather is the general trade term for a break in a gemstone. It often looks white and feathery.
- **[Fingerprint](https://digitalgemology.com/learn/glossary/fingerprint)**: 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.
- **[Fissure](https://digitalgemology.com/learn/glossary/fissure)**: A fissure is a crack in a gem, often open to the surface. Labs use the word for the cracks that fillers can enter.
- **[Grain center](https://digitalgemology.com/learn/glossary/grain-center)**: 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.
- **[Inclusion](https://digitalgemology.com/learn/glossary/inclusion)**: An inclusion is a clarity characteristic inside a diamond, or one that reaches the surface from inside.
- **[Indented natural](https://digitalgemology.com/learn/glossary/indented-natural)**: An indented natural is a part of the rough diamond's original surface that dips below the polished surface.
- **[Internal graining](https://digitalgemology.com/learn/glossary/internal-graining)**: 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.
- **[Internal laser drilling](https://digitalgemology.com/learn/glossary/internal-laser-drilling)**: 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.
- **[Jardin](https://digitalgemology.com/learn/glossary/jardin)**: Jardin, French for garden, is a trade word for the mossy, garden-like inclusions typical of emerald.
- **[Knot](https://digitalgemology.com/learn/glossary/knot)**: A knot is a white or transparent diamond crystal that reaches the surface after the stone is cut.
- **[Laser drill hole](https://digitalgemology.com/learn/glossary/laser-drill-hole)**: A laser drill hole is a tiny, surface-reaching tunnel made by a laser beam, usually toward a dark inclusion.
- **[Lizard skin](https://digitalgemology.com/learn/glossary/lizard-skin)**: Lizard skin is a wavy or bumpy area on the surface of a polished diamond.
- **[Natural](https://digitalgemology.com/learn/glossary/natural)**: 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.
- **[Needle](https://digitalgemology.com/learn/glossary/needle)**: A needle is a thin, elongated crystal inclusion that looks like a tiny rod at 10x.
- **[Nick](https://digitalgemology.com/learn/glossary/nick)**: 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.
- **[Pinpoint](https://digitalgemology.com/learn/glossary/pinpoint)**: A pinpoint is a very small crystal inside a diamond that looks like a tiny dot at 10x.
- **[Pit](https://digitalgemology.com/learn/glossary/pit)**: A pit is a small opening in the surface that looks like a tiny white dot.
- **[Polish lines](https://digitalgemology.com/learn/glossary/polish-lines)**: 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.
- **[Rough girdle](https://digitalgemology.com/learn/glossary/rough-girdle)**: A rough girdle has an irregular or granular girdle surface.
- **[Scratch](https://digitalgemology.com/learn/glossary/scratch)**: A scratch is a thin, dull white line across the surface with no clear depth at 10x.
- **[Silk](https://digitalgemology.com/learn/glossary/silk)**: Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.
- **[Surface graining](https://digitalgemology.com/learn/glossary/surface-graining)**: 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.
- **[Three-phase inclusion](https://digitalgemology.com/learn/glossary/three-phase-inclusion)**: 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).
- **[Twinning wisp](https://digitalgemology.com/learn/glossary/twinning-wisp)**: 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.

## Color and fluorescence

- **[Colorless (D–F)](https://digitalgemology.com/learn/glossary/colorless)**: Colorless is the top range of the D-to-Z scale: the grades D, E and F.
- **[Faint (K–M)](https://digitalgemology.com/learn/glossary/faint-color)**: Faint is the D-to-Z range from K to M: a faint yellow or brown tint.
- **[Fancy color diamond](https://digitalgemology.com/learn/glossary/fancy-color-diamond)**: 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.
- **[Fancy color grade](https://digitalgemology.com/learn/glossary/fancy-color-grade)**: 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.
- **[Fluorescence](https://digitalgemology.com/learn/glossary/fluorescence)**: 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.
- **[Hue, tone and saturation](https://digitalgemology.com/learn/glossary/hue-tone-saturation)**: 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.
- **[Light (S–Z)](https://digitalgemology.com/learn/glossary/light-color)**: Light is the D-to-Z range from S to Z: a plainly visible yellow or brown tint.
- **[Master stones](https://digitalgemology.com/learn/glossary/master-stones)**: Master stones are reference diamonds of known color grade. A grader sets a diamond beside them to decide its D-to-Z letter.
- **[Near colorless (G–J)](https://digitalgemology.com/learn/glossary/near-colorless)**: Near colorless is the D-to-Z range from G to J.
- **[Very light (N–R)](https://digitalgemology.com/learn/glossary/very-light-color)**: Very light is the D-to-Z range from N to R: a yellow or brown tint that is easy to see.

## Light performance

- **[Brilliance](https://digitalgemology.com/learn/glossary/brilliance)**: 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.
- **[Contrast (Light Return)](https://digitalgemology.com/learn/glossary/contrast)**: 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.
- **[Direct light (Light Return)](https://digitalgemology.com/learn/glossary/direct-light)**: Direct light is light that reaches the viewer from 45° to 75° above the girdle plane. Our light map shows it in red.
- **[Fire](https://digitalgemology.com/learn/glossary/fire)**: 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.
- **[Head-on light](https://digitalgemology.com/learn/glossary/head-on-light)**: 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.
- **[Hearts and arrows](https://digitalgemology.com/learn/glossary/hearts-and-arrows)**: 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.
- **[Indirect light (Light Return)](https://digitalgemology.com/learn/glossary/indirect-light)**: Indirect light is light that reaches the viewer from below 45° above the girdle plane. Our light map shows it in green.
- **[Leakage](https://digitalgemology.com/learn/glossary/leakage)**: Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.
- **[Light performance](https://digitalgemology.com/learn/glossary/light-performance)**: Light performance is how well a cut stone handles light. We describe it with brilliance (light returned), fire (color) and scintillation (sparkle).
- **[Light Return](https://digitalgemology.com/learn/glossary/light-return)**: 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.
- **[Overall score](https://digitalgemology.com/learn/glossary/overall-score)**: 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.
- **[Scintillation](https://digitalgemology.com/learn/glossary/scintillation)**: 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.
- **[Shape-norm](https://digitalgemology.com/learn/glossary/shape-norm)**: 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.
- **[Spread (light performance)](https://digitalgemology.com/learn/glossary/spread-score)**: 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.
- **[Symmetry (light performance)](https://digitalgemology.com/learn/glossary/symmetry-light-performance)**: 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.
- **[Tilt (light performance)](https://digitalgemology.com/learn/glossary/tilt-light-performance)**: 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.
- **[Windowing](https://digitalgemology.com/learn/glossary/windowing)**: 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.

## How we measure

- **[3D Gemstone Simulator](https://digitalgemology.com/learn/glossary/3d-gemstone-simulator)**: 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.
- **[Brightness & Tilt Analysis](https://digitalgemology.com/learn/glossary/brightness-and-tilt-analysis)**: 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.
- **[Census ray classes](https://digitalgemology.com/learn/glossary/census-ray-classes)**: 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.
- **[COS lighting](https://digitalgemology.com/learn/glossary/cos-lighting)**: 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.
- **[Cut Grading Tool](https://digitalgemology.com/learn/glossary/cut-grading-tool)**: 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.
- **[Cut Performance Index](https://digitalgemology.com/learn/glossary/cut-performance-index)**: 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.
- **[Cut Performance Wheel](https://digitalgemology.com/learn/glossary/cut-performance-wheel)**: 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.
- **[Cutting-file accuracy](https://digitalgemology.com/learn/glossary/cutting-file-accuracy)**: 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.
- **[Dome model](https://digitalgemology.com/learn/glossary/dome-model)**: 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.
- **[Face-up size](https://digitalgemology.com/learn/glossary/face-up-size)**: 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.
- **[Fire spread (Δ)](https://digitalgemology.com/learn/glossary/fire-spread)**: 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.
- **[Fixed settings](https://digitalgemology.com/learn/glossary/fixed-settings)**: 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.
- **[Head shadow](https://digitalgemology.com/learn/glossary/head-shadow)**: 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.
- **[Hearts & Arrows Scope](https://digitalgemology.com/learn/glossary/hearts-and-arrows-scope)**: 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.
- **[Hearts and arrows symmetry](https://digitalgemology.com/learn/glossary/hearts-and-arrows-symmetry)**: 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.
- **[Hearts and arrows viewer](https://digitalgemology.com/learn/glossary/hearts-and-arrows-viewer)**: 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.
- **[ISO lighting](https://digitalgemology.com/learn/glossary/iso-lighting)**: 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.
- **[Light census](https://digitalgemology.com/learn/glossary/light-census)**: 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.
- **[Light map](https://digitalgemology.com/learn/glossary/light-map)**: 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.
- **[Measure your own stone](https://digitalgemology.com/learn/glossary/measure-your-own-stone)**: “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.
- **[Ray tracing](https://digitalgemology.com/learn/glossary/ray-tracing)**: 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.
- **[SC2 lighting](https://digitalgemology.com/learn/glossary/sc2-lighting)**: SC2 is a dome model with a ring of light that peaks at 45° elevation. It is dark overhead and at the horizon.
- **[Schlick's approximation](https://digitalgemology.com/learn/glossary/schlick-approximation)**: 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.
- **[Simulator cut grade](https://digitalgemology.com/learn/glossary/simulator-cut-grade)**: 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.
- **[T73 benchmark](https://digitalgemology.com/learn/glossary/t73-benchmark)**: 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.
- **[Weight from volume](https://digitalgemology.com/learn/glossary/weight-from-volume)**: 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.

## Optics

- **[Absorption line](https://digitalgemology.com/learn/glossary/absorption-line)**: 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.
- **[Absorption spectrum](https://digitalgemology.com/learn/glossary/absorption-spectrum)**: An absorption spectrum shows which wavelengths of light a stone absorbs. In a spectroscope, the absorbed parts of the spectrum look dark.
- **[Beer–Lambert law](https://digitalgemology.com/learn/glossary/beer-lambert-law)**: 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.
- **[Biaxial](https://digitalgemology.com/learn/glossary/biaxial)**: 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.
- **[Birefringence](https://digitalgemology.com/learn/glossary/birefringence)**: 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.
- **[CIE colorimetry](https://digitalgemology.com/learn/glossary/cie-colorimetry)**: 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.
- **[CIEDE2000 color difference](https://digitalgemology.com/learn/glossary/ciede2000)**: CIEDE2000 (ΔE00) is a standard formula for how different two colors look to a human observer. A larger number means a larger visible difference.
- **[Critical angle](https://digitalgemology.com/learn/glossary/critical-angle)**: 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).
- **[Dispersion](https://digitalgemology.com/learn/glossary/dispersion)**: 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).
- **[Doubly refractive](https://digitalgemology.com/learn/glossary/doubly-refractive)**: 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.
- **[Extraordinary ray](https://digitalgemology.com/learn/glossary/extraordinary-ray)**: 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.
- **[Fresnel reflection](https://digitalgemology.com/learn/glossary/fresnel-reflection)**: 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.
- **[High-order white](https://digitalgemology.com/learn/glossary/high-order-white)**: 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.
- **[Interference figure](https://digitalgemology.com/learn/glossary/interference-figure)**: 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.
- **[Jones calculus](https://digitalgemology.com/learn/glossary/jones-calculus)**: Jones calculus is a matrix method for following polarized light through filters, retarders and crystals. R.C. Jones published it in 1941.
- **[Optic axis](https://digitalgemology.com/learn/glossary/optic-axis)**: 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.
- **[Optic sign](https://digitalgemology.com/learn/glossary/optic-sign)**: 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.
- **[Optical activity](https://digitalgemology.com/learn/glossary/optical-activity)**: 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.
- **[Ordinary ray](https://digitalgemology.com/learn/glossary/ordinary-ray)**: 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.
- **[Pleochroism](https://digitalgemology.com/learn/glossary/pleochroism)**: 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.
- **[Polarized light](https://digitalgemology.com/learn/glossary/polarized-light)**: 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.
- **[Refractive index](https://digitalgemology.com/learn/glossary/refractive-index)**: 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.
- **[Singly refractive](https://digitalgemology.com/learn/glossary/singly-refractive)**: 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.
- **[Snell's law](https://digitalgemology.com/learn/glossary/snells-law)**: 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.
- **[Total internal reflection](https://digitalgemology.com/learn/glossary/total-internal-reflection)**: 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.
- **[Uniaxial](https://digitalgemology.com/learn/glossary/uniaxial)**: A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.

## Gem testing

- **[Blink (polariscope)](https://digitalgemology.com/learn/glossary/blink-polariscope)**: 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.
- **[Conoscope](https://digitalgemology.com/learn/glossary/conoscope)**: 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.
- **[Contact liquid](https://digitalgemology.com/learn/glossary/contact-liquid)**: 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.
- **[Dichroscope](https://digitalgemology.com/learn/glossary/dichroscope)**: 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.
- **[Electrical conductivity tester](https://digitalgemology.com/learn/glossary/electrical-conductivity-tester)**: An electrical conductivity tester checks whether a stone conducts electricity. Many moissanites conduct, and most near-colorless diamonds do not.
- **[Facet doubling](https://digitalgemology.com/learn/glossary/facet-doubling)**: 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.
- **[First-order red plate](https://digitalgemology.com/learn/glossary/red-plate)**: 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.
- **[Heavy liquid](https://digitalgemology.com/learn/glossary/heavy-liquid)**: A heavy liquid is a dense liquid that sorts stones by specific gravity. A lighter stone floats in it, and a denser one sinks.
- **[Over the limit (OTL)](https://digitalgemology.com/learn/glossary/over-the-limit)**: 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.
- **[Polariscope](https://digitalgemology.com/learn/glossary/polariscope)**: 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.
- **[Raman spectroscopy](https://digitalgemology.com/learn/glossary/raman-spectroscopy)**: Raman spectroscopy is a laboratory method that identifies a material from how it scatters laser light. Diamond shows a peak at 1332 cm⁻¹.
- **[Refractometer](https://digitalgemology.com/learn/glossary/refractometer)**: 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.
- **[Screening device](https://digitalgemology.com/learn/glossary/screening-device)**: A screening device separates natural diamonds from laboratory-grown diamonds and simulants. Most answer Pass (natural) or Refer (send to a laboratory).
- **[Shadow edge](https://digitalgemology.com/learn/glossary/shadow-edge)**: 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.
- **[Spectroscope](https://digitalgemology.com/learn/glossary/spectroscope)**: 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.
- **[Strain](https://digitalgemology.com/learn/glossary/strain)**: 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.
- **[Thermal diamond tester](https://digitalgemology.com/learn/glossary/thermal-diamond-tester)**: A thermal diamond tester measures how fast a stone draws heat from a hot tip. Diamond conducts heat very well, so it reads “diamond”.

## Gemstone properties

- **[Asterism](https://digitalgemology.com/learn/glossary/asterism)**: 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.
- **[Chatoyancy](https://digitalgemology.com/learn/glossary/chatoyancy)**: 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.
- **[Cleavage](https://digitalgemology.com/learn/glossary/cleavage)**: 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.
- **[Color change](https://digitalgemology.com/learn/glossary/color-change)**: Color change is a visible change of a gem's main hue between daylight and incandescent light.
- **[Crystal system](https://digitalgemology.com/learn/glossary/crystal-system)**: A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.
- **[Durability](https://digitalgemology.com/learn/glossary/durability)**: 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.
- **[Fracture](https://digitalgemology.com/learn/glossary/fracture)**: Fracture is the way a gem breaks when it does not split along a cleavage plane. Conchoidal fracture leaves curved, shell-like surfaces.
- **[Guide minimum pavilion angle](https://digitalgemology.com/learn/glossary/guide-minimum-pavilion-angle)**: 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.
- **[Hardness (Mohs)](https://digitalgemology.com/learn/glossary/hardness)**: Hardness is a gem's resistance to scratching. Gemologists rank it on the Mohs scale, on which diamond is 10.
- **[Luster](https://digitalgemology.com/learn/glossary/luster)**: Luster is the way the surface of a stone reflects light. It depends mostly on the refractive index and the quality of the polish.
- **[Mohs scale](https://digitalgemology.com/learn/glossary/mohs-scale)**: The Mohs scale ranks minerals from 1 to 10 by which can scratch which. Diamond is 10 and corundum is 9.
- **[Specific gravity](https://digitalgemology.com/learn/glossary/specific-gravity)**: 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.
- **[Stability](https://digitalgemology.com/learn/glossary/stability)**: Stability is a gem's resistance to damage from heat, light and chemicals.
- **[Toughness](https://digitalgemology.com/learn/glossary/toughness)**: Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.
- **[Trapiche](https://digitalgemology.com/learn/glossary/trapiche)**: 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.

## Colored gemstones

- **[Amethyst](https://digitalgemology.com/learn/glossary/amethyst)**: Amethyst is the purple variety of quartz. Iron in the crystal, together with natural radiation, causes its color.
- **[Ametrine](https://digitalgemology.com/learn/glossary/ametrine)**: Ametrine is quartz that is part amethyst and part citrine: purple and yellow zones in one crystal.
- **[Beryl](https://digitalgemology.com/learn/glossary/beryl)**: Beryl is the mineral species of emerald. Aquamarine and the other beryl colors belong to the same species.
- **[Citrine](https://digitalgemology.com/learn/glossary/citrine)**: Citrine is the pale yellow to brownish orange variety of quartz.
- **[Color zoning](https://digitalgemology.com/learn/glossary/color-zoning)**: Color zoning is uneven color in a gem: bands or patches of deeper and paler color.
- **[Corundum](https://digitalgemology.com/learn/glossary/corundum)**: Corundum is the mineral species made only of aluminum and oxygen. Its red variety is ruby, and all its other colors are sapphire.
- **[Emerald](https://digitalgemology.com/learn/glossary/emerald)**: Emerald is the green to bluish green variety of the mineral beryl. Chromium, vanadium or both cause its color.
- **[Fancy sapphire](https://digitalgemology.com/learn/glossary/fancy-sapphire)**: A fancy sapphire is a sapphire of any color other than blue, such as yellow, orange, pink, purple or green.
- **[Gem species](https://digitalgemology.com/learn/glossary/gem-species)**: A gem species is a mineral with one chemical composition and one crystal structure, such as corundum, beryl or quartz.
- **[Gem variety](https://digitalgemology.com/learn/glossary/gem-variety)**: 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.
- **[Green beryl](https://digitalgemology.com/learn/glossary/green-beryl)**: 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.
- **[Hue](https://digitalgemology.com/learn/glossary/hue)**: Hue is the basic color of a gem, such as red, blue or green. A second color can modify it, as in purplish red.
- **[Kashmir sapphire](https://digitalgemology.com/learn/glossary/kashmir-sapphire)**: 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.
- **[Padparadscha](https://digitalgemology.com/learn/glossary/padparadscha)**: Padparadscha is a sapphire of soft pinkish orange to orangey pink. Its tones are pastel and its saturation low to medium.
- **[Pigeon's blood](https://digitalgemology.com/learn/glossary/pigeons-blood)**: Pigeon's blood is a trade name for a ruby of intense, saturated, even red. It has no single agreed standard.
- **[Pink sapphire](https://digitalgemology.com/learn/glossary/pink-sapphire)**: Pink sapphire is corundum whose color is pink rather than red. Chromium causes both its pink and the red of ruby.
- **[Quartz](https://digitalgemology.com/learn/glossary/quartz)**: Quartz is the mineral species of amethyst and citrine. It is silicon dioxide in a trigonal crystal.
- **[Royal blue](https://digitalgemology.com/learn/glossary/royal-blue)**: Royal blue is a trade name for a sapphire of intense, saturated, even blue. It is not a single agreed standard.
- **[Ruby](https://digitalgemology.com/learn/glossary/ruby)**: Ruby is the red variety of the mineral corundum. Chromium causes its red color.
- **[Sapphire](https://digitalgemology.com/learn/glossary/sapphire)**: 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.
- **[Saturation](https://digitalgemology.com/learn/glossary/saturation)**: Saturation is the strength or purity of a gem's color, from grayish or brownish to vivid.
- **[Tone](https://digitalgemology.com/learn/glossary/tone)**: Tone is how light or dark a gem's color is, from very light to very dark.
- **[Trade color term](https://digitalgemology.com/learn/glossary/trade-color-term)**: 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.

## Treatments and origin

- **[Assembled stone](https://digitalgemology.com/learn/glossary/assembled-stone)**: 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.
- **[Beryllium diffusion](https://digitalgemology.com/learn/glossary/beryllium-diffusion)**: Beryllium diffusion is heating corundum with an added element, such as beryllium. The element diffuses into the crystal and changes its color.
- **[Clarity enhancement](https://digitalgemology.com/learn/glossary/clarity-enhancement)**: Clarity enhancement is filling surface-reaching fissures with a filler such as oil, resin, wax or glass, so they show less.
- **[Coating](https://digitalgemology.com/learn/glossary/coating)**: Coating is a thin film, paint or ink on the surface of a gem that changes its color or look.
- **[Color treatment](https://digitalgemology.com/learn/glossary/color-treatment)**: A color treatment is any process that changes a diamond's color, such as HPHT processing or coating.
- **[Colored stone report](https://digitalgemology.com/learn/glossary/colored-stone-report)**: 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.
- **[CVD](https://digitalgemology.com/learn/glossary/cvd)**: CVD, chemical vapor deposition, is one of the two usual methods for growing diamond in a laboratory. The other is HPHT.
- **[Czochralski growth](https://digitalgemology.com/learn/glossary/czochralski-growth)**: 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.
- **[Disclosure](https://digitalgemology.com/learn/glossary/disclosure)**: Disclosure is telling the buyer, before the sale, that a gem is treated, laboratory-grown or an imitation.
- **[Dyeing](https://digitalgemology.com/learn/glossary/dyeing)**: Dyeing is adding a colored substance to a gem's fractures or pores to change its color.
- **[Emerald oiling](https://digitalgemology.com/learn/glossary/emerald-oiling)**: Emerald oiling is filling an emerald's fissures with a colorless oil to make them less visible.
- **[Enhancement degree](https://digitalgemology.com/learn/glossary/enhancement-degree)**: The enhancement degree is the amount of filler a lab finds in a gem's fissures: minor, moderate or significant.
- **[Flame fusion](https://digitalgemology.com/learn/glossary/flame-fusion)**: 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.
- **[Flux growth](https://digitalgemology.com/learn/glossary/flux-growth)**: Flux growth is a way to grow synthetic crystals from a chemical solvent (flux). Crystals form as the hot solution slowly cools.
- **[Fracture filling](https://digitalgemology.com/learn/glossary/fracture-filling)**: Fracture filling is a clarity treatment that injects a glass-like filler into a crack to make it less visible.
- **[Heat treatment](https://digitalgemology.com/learn/glossary/heat-treatment)**: Heat treatment is controlled heating of a gem to change its color or clarity.
- **[HPHT](https://digitalgemology.com/learn/glossary/hpht)**: 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.
- **[Hydrothermal growth](https://digitalgemology.com/learn/glossary/hydrothermal-growth)**: Hydrothermal growth makes synthetic crystals in a hot water solution under high pressure.
- **[Indications of heating](https://digitalgemology.com/learn/glossary/indications-of-heating)**: '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.
- **[Irradiation](https://digitalgemology.com/learn/glossary/irradiation)**: Irradiation is exposing a gem to artificial radiation to change its color. Heating sometimes follows, to modify the color further.
- **[Laboratory-grown diamond](https://digitalgemology.com/learn/glossary/laboratory-grown-diamond)**: 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.
- **[Laser drilling](https://digitalgemology.com/learn/glossary/laser-drilling)**: 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.
- **[Lead-glass filling](https://digitalgemology.com/learn/glossary/lead-glass-filling)**: Lead-glass filling is a treatment that fills fissures and cavities in corundum, mostly ruby, with high-lead glass to hide them.
- **[Manufactured product](https://digitalgemology.com/learn/glossary/manufactured-product)**: 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.
- **[Natural diamond](https://digitalgemology.com/learn/glossary/natural-diamond)**: 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.
- **[No indications of heating](https://digitalgemology.com/learn/glossary/no-indications-of-heating)**: 'No indications of heating' (NH) is the standard lab wording for a corundum that shows no signs of heat treatment.
- **[Origin report](https://digitalgemology.com/learn/glossary/origin-report)**: An origin report gives a lab's opinion on the country or region where a gem was mined.
- **[Residues in healed fissures](https://digitalgemology.com/learn/glossary/residues-in-healed-fissures)**: Residues in healed fissures are remains of the heating process, often glassy. They stay in fissures that healed while a corundum was heated.
- **[Resin filling](https://digitalgemology.com/learn/glossary/resin-filling)**: Resin filling is filling a gem's fissures or cavities with a resin, often one that hardens, to hide them.
- **[Skull melting](https://digitalgemology.com/learn/glossary/skull-melting)**: 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.
- **[Synthetic gemstone](https://digitalgemology.com/learn/glossary/synthetic-gemstone)**: 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.
- **[Treatment](https://digitalgemology.com/learn/glossary/treatment)**: 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.

## Diamond and simulants

- **[Cubic zirconia](https://digitalgemology.com/learn/glossary/cubic-zirconia)**: Cubic zirconia (CZ) is a man-made diamond simulant. GIA names it as the most common simulant today.
- **[Diamond](https://digitalgemology.com/learn/glossary/diamond)**: Diamond is a single element, carbon, with every atom bonded to four neighbors in a cubic crystal. It is the hardest gem, Mohs 10.
- **[Moissanite](https://digitalgemology.com/learn/glossary/moissanite)**: 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.
- **[Simulant](https://digitalgemology.com/learn/glossary/simulant)**: 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.
- **[Type IIb diamond](https://digitalgemology.com/learn/glossary/type-iib-diamond)**: A type IIb diamond contains boron, which can make it blue or gray. The boron also lets it conduct electricity.

## Cuts and shapes

- **[Asscher cut](https://digitalgemology.com/learn/glossary/asscher-cut)**: An Asscher cut is a square step cut with deeply cropped corners, which give it an octagonal outline.
- **[Bow-tie](https://digitalgemology.com/learn/glossary/bow-tie)**: 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.
- **[Brilliant cut](https://digitalgemology.com/learn/glossary/brilliant-cut)**: 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.
- **[Bruting](https://digitalgemology.com/learn/glossary/bruting)**: Bruting is shaping the outline of a diamond, usually round, by grinding it against another diamond.
- **[Cushion cut](https://digitalgemology.com/learn/glossary/cushion-cut)**: A cushion cut is a square outline with rounded corners and a brilliant facet plan. It developed from the old mine cut.
- **[Emerald cut](https://digitalgemology.com/learn/glossary/emerald-cut)**: An emerald cut is a rectangular step cut with cropped corners and a long, open table.
- **[Fancy shape](https://digitalgemology.com/learn/glossary/fancy-shape)**: A fancy shape is any diamond outline other than round, such as oval, pear, marquise, cushion, princess or emerald.
- **[Fish-eye](https://digitalgemology.com/learn/glossary/fish-eye)**: 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.
- **[Marquise cut](https://digitalgemology.com/learn/glossary/marquise-cut)**: A marquise cut is an elongated brilliant pointed at both ends.
- **[Mazarin cut](https://digitalgemology.com/learn/glossary/mazarin-cut)**: 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.
- **[Old European cut](https://digitalgemology.com/learn/glossary/old-european-cut)**: 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.
- **[Old mine cut](https://digitalgemology.com/learn/glossary/old-mine-cut)**: 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.
- **[Old single cut](https://digitalgemology.com/learn/glossary/old-single-cut)**: 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.
- **[Oval cut](https://digitalgemology.com/learn/glossary/oval-cut)**: An oval cut is an elongated round: the brilliant facet plan stretched along one axis.
- **[Pear cut](https://digitalgemology.com/learn/glossary/pear-cut)**: A pear cut is a brilliant with one rounded end and one point, halfway between an oval and a marquise.
- **[Peruzzi cut](https://digitalgemology.com/learn/glossary/peruzzi-cut)**: 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.
- **[Princess cut](https://digitalgemology.com/learn/glossary/princess-cut)**: A princess cut is a square brilliant with sharp corners and chevron-patterned pavilion facets.
- **[Radiant cut](https://digitalgemology.com/learn/glossary/radiant-cut)**: A radiant cut has the outline of an emerald cut, with cropped corners, and a brilliant facet plan.
- **[Rose cut](https://digitalgemology.com/learn/glossary/rose-cut)**: A rose cut is a dome of triangular facets on a flat base: all crown and no pavilion.
- **[Round brilliant](https://digitalgemology.com/learn/glossary/round-brilliant)**: 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.
- **[Step cut](https://digitalgemology.com/learn/glossary/step-cut)**: 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.
- **[Tolkowsky ideal cut](https://digitalgemology.com/learn/glossary/tolkowsky-ideal-cut)**: 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.

## Cutting and files

- **[3D model](https://digitalgemology.com/learn/glossary/3d-model)**: 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.
- **[3DM file](https://digitalgemology.com/learn/glossary/3dm-file)**: 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.
- **[Convex](https://digitalgemology.com/learn/glossary/convex)**: 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.
- **[Cutting file](https://digitalgemology.com/learn/glossary/cutting-file)**: 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.
- **[Cutting order](https://digitalgemology.com/learn/glossary/cutting-order)**: 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.
- **[Facet](https://digitalgemology.com/learn/glossary/facet)**: A facet is one flat, polished face of a cut gemstone. Its angle and its direction around the stone decide where it sends light.
- **[Facet count](https://digitalgemology.com/learn/glossary/facet-count)**: 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.
- **[Faceting diagram](https://digitalgemology.com/learn/glossary/faceting-diagram)**: 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.
- **[Faceting machine](https://digitalgemology.com/learn/glossary/faceting-machine)**: 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.
- **[FBX file](https://digitalgemology.com/learn/glossary/fbx-file)**: FBX is a 3D file format for animation and game engines such as Unity and Unreal Engine.
- **[Gem Cut Studio](https://digitalgemology.com/learn/glossary/gem-cut-studio)**: 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.
- **[GemCad .asc](https://digitalgemology.com/learn/glossary/gemcad-asc)**: 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.
- **[GemRay](https://digitalgemology.com/learn/glossary/gemray)**: 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.
- **[GLB file](https://digitalgemology.com/learn/glossary/glb-file)**: 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.
- **[Index gear](https://digitalgemology.com/learn/glossary/index-gear)**: 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.
- **[Meetpoint](https://digitalgemology.com/learn/glossary/meetpoint)**: 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.
- **[Mesh](https://digitalgemology.com/learn/glossary/mesh)**: A mesh describes a 3D surface as many small flat triangles joined at their edges. STL, OBJ, FBX and GLB files store a mesh.
- **[Mirror-image symmetry (design)](https://digitalgemology.com/learn/glossary/mirror-image-symmetry)**: 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.
- **[OBJ file](https://digitalgemology.com/learn/glossary/obj-file)**: OBJ is a mesh format that almost every 3D program reads, including Blender, ZBrush, Maya and Rhino.
- **[Planar facet](https://digitalgemology.com/learn/glossary/planar-facet)**: 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.
- **[Reference size](https://digitalgemology.com/learn/glossary/reference-size)**: 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.
- **[STEP file](https://digitalgemology.com/learn/glossary/step-file)**: 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.
- **[STL file](https://digitalgemology.com/learn/glossary/stl-file)**: STL is a triangle-mesh format and the standard for 3D printing. Slicers such as PrusaSlicer, Cura and Chitubox read it.
- **[Tier](https://digitalgemology.com/learn/glossary/tier)**: 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.
- **[Tier table](https://digitalgemology.com/learn/glossary/tier-table)**: 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.
- **[Turned girdle](https://digitalgemology.com/learn/glossary/turned-girdle)**: 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.
- **[Watertight](https://digitalgemology.com/learn/glossary/watertight)**: 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.

## For AI assistants

The glossary is also an MCP server: `https://digitalgemology.com/mcp/glossary`, free and with no key. As JSON: [/api/glossary.json](https://digitalgemology.com/api/glossary.json). See [Connect your AI](https://digitalgemology.com/about/connect-ai).