--- title: "Octagon Cut Spinel: Pros, Cons and Scores" description: "Octagon Cut in spinel: brilliance 78, fire 41, leakage 21%, 0 pros and 4 cons against the T73, from our ray-traced light census." url: https://digitalgemology.com/gemstones/spinel/octagon image: https://digitalgemology.com/img/stone-cuts/octagon-spinel-og.jpg?v=c2cc529c --- > Markdown copy of https://digitalgemology.com/gemstones/spinel/octagon. 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 # Octagon Cut Spinel: what our light census measures **Buy (instant download, personal-use license):** [Octagon Cut 3D files, €9.99 · Personal use](https://digitalgemology.lemonsqueezy.com/checkout/buy/26baac35-16bd-4143-97ba-9eeb7fd4da11) · [Octagon Cut cutting files, €6.99 · Personal use](https://digitalgemology.lemonsqueezy.com/checkout/buy/390b595d-894b-4e34-be35-57f44d569591) · [Buy the bundle](https://digitalgemology.lemonsqueezy.com/checkout/buy/9170b0c0-5357-490e-b6ef-3ce4f7fcd282?checkout%5Bdiscount_code%5D=BUNDLE25) · [Get the cube free](https://digitalgemology.lemonsqueezy.com/checkout/buy/2d1ab55f-2bd6-4897-a4cd-f3320f54e99e) Brilliance 78 · Fire 41 · Scintillation 66 · in our census, where the T73 scores 94 · 47 · 80 in spinel How the Octagon Cut handles light in spinel, measured: brilliance 78, fire 41 and 21% leakage, set against the T73 in the same stone and against the same cut in diamond. [Get the files](https://digitalgemology.com/gemstones/spinel/octagon#buy) [Open it in the Simulator](https://digitalgemology.com/tools/3d_gemstone_simulator?cut=octagon&stone=spinel&ct=1.00&view=angle&lighting=studio&turn=0) [All cuts in spinel](https://digitalgemology.com/gemstones/spinel) Light Return map of the Octagon Cut in spinel, face-up, from our own tracer at refractive index 1.712. Red, green and blue return light to the eye; gray leaks. ## Octagon Cut Spinel Spinel has a refractive index of 1.712 to 1.747 and a dispersion of 0.020. Its critical angle, from the low end of that range, is 35.7°, so a round design needs a pavilion of at least 36.1° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.730, the midpoint of the published range, and a fire spread of 0.017, 0.86 of the published dispersion. Every picture here is our Light Return tracer at 1.712, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the Octagon Cut in spinel live. The Octagon Cut has 49 facets, a table of 60.0% of its width and a total depth of 61.0% of its width; its pavilion mains sit at 29.3°. In spinel, the census returns 78 of 100 for brilliance and 41 for fire, against 94 and 47 for the T73, our benchmark, in the same stone; the same Octagon Cut in diamond reads 71 and 76. ## Is spinel good in an Octagon? Pros and cons, measured ### Pros No figure ahead by our thresholds ### Cons - [Less fire in spinel than in diamond](https://digitalgemology.com/gemstones/spinel/octagon#con-fire) 41 vs 76 - [Returns less light than the T73 in spinel](https://digitalgemology.com/gemstones/spinel/octagon#con-brilliance) 78 vs 94 - [Leaks more light than the T73 in spinel](https://digitalgemology.com/gemstones/spinel/octagon#con-leak) 21 vs 6 - [Fewer facets, less sparkle](https://digitalgemology.com/gemstones/spinel/octagon#con-scint) 66 vs 80 Compared with the T73 Round Brilliant in spinel and with the Octagon Cut in diamond. A difference counts from 3 points (Brilliance, Fire, Tilt, Leakage), 5 (Scintillation, Symmetry), 4% (face-up size per carat) or 5% (weight); Tilt counts as a pro only for a stone within 5 points of the brilliance it is compared with. [How we measure](https://digitalgemology.com/about/how-we-measure). ## All eight scores against the T73 At a glance: the Octagon Cut in spinel (white) against the T73 Round Brilliant in spinel (dashed): solid white where it reaches further, hatched where it falls short. Each pro and con is explained, with pictures, below. Against the T73 Round Brilliant in spinel: ahead on 2, level on 2, behind on 4. - Global 73, the comparison 81, difference −8 - Brilliance 78, the comparison 94, difference −16 - Fire 41, the comparison 47, difference −6 - Scintillation 66, the comparison 80, difference −14 - Spread 100, the comparison 100, difference 0 - Symmetry 100, the comparison 100, difference 0 - Shape-norm 102, the comparison 101, difference +1 - Tilt 86, the comparison 75, difference +11 Face-up light census, a different measure from the Cut Grading Tool; Spread and Shape-norm above 100 sit on the ring. Spread is the census's estimate from depth alone; the face-up size in the pros and cons is measured from each model's outline. [Open it in the Cut Performance Wheel](https://digitalgemology.com/tools/cut-performance-wheel?cut=octagon&stone=spinel); these are the same census figures as our [Cut Performance Index](https://digitalgemology.com/learn/cut-performance-index). ## What the Octagon Cut gains in spinel Against the T73 in spinel and the same cut in diamond, no figure of the Octagon Cut is ahead by the thresholds above. Its figures are in the wheel and the FAQ; [Spinel](https://digitalgemology.com/gemstones/spinel) ranks every cut we score in spinel. ## What the Octagon Cut gives up in spinel ### Less fire in spinel than in diamond **Fire 41** against 76 for the Octagon Cut in diamond: 35 points lower Fire is the share of the returned light that splits into color at the exit facet: the census traces each exit again at the refractive index minus and plus the fire spread (0.017 in spinel) and counts a ray as fire when the two directions differ by more than 0.6°. The Octagon Cut reads 41 against 76 for the same cut in diamond. The figure is the census's; the 3D Gemstone Simulator renders spinel's fire live. The fire figure is the census's, traced at the refractive index minus and plus the fire spread. [See the fire in the 3D Gemstone Simulator](https://digitalgemology.com/tools/3d_gemstone_simulator?cut=octagon&stone=spinel&ct=1.00&view=up&lighting=sun&turn=0): the Octagon Cut in spinel under the sunny-day lighting. **What helps:** [the DG-Elongated Hexagon](https://digitalgemology.com/gemstones/spinel/dg-elongated-hexagon) reads 80 for fire in spinel, the best of the cuts we score there. [Try it yourself: See the fire in the 3D Gemstone Simulator](https://digitalgemology.com/tools/3d_gemstone_simulator?cut=octagon&stone=spinel&ct=1.00&view=up&lighting=sun&turn=0) ### Returns less light than the T73 in spinel **Brilliance 78** against 94 for the T73 Round Brilliant in spinel: 16 points lower Brilliance is the share of the 3,200 census rays that enter the stone and come back out of the crown towards the viewer. In spinel, whose critical angle is 35.7°, the Octagon Cut returns 78 of 100 and the T73 94. The Octagon Cut's pavilion mains sit at 29.3°, the T73's at 40.8°. The maps below show where each stone's light returns from and where it leaks. Octagon Cut in spinel · Brilliance 78 T73 Round Brilliant in spinel · Brilliance 94 Red, green and blue = light returning to the eye; gray = leakage, light that leaves heading down or stays inside. This is the head-on view; the Brilliance score averages a wider cone of light, so the score can differ from what one picture shows. **What helps:** [the DG-M5](https://digitalgemology.com/gemstones/spinel/dg-m5) reads 100 for brilliance in spinel, the best of the cuts we score there. [Try it yourself: Open this Light Return map in the simulator](https://digitalgemology.com/tools/3d_gemstone_simulator?cut=octagon&stone=spinel&ct=1.00&view=up&map=1&turn=0) ### Leaks more light than the T73 in spinel **Leakage 21** against 6 for the T73 Round Brilliant in spinel: 15 points higher Leakage is the share of the census rays that enter the stone and leave through the pavilion, or through the crown heading down. The Octagon Cut leaks 21% in spinel against 6% for the T73 in spinel. Grey in the maps below is leaking light; the share under each picture is that picture's own. Octagon Cut in spinel · Leakage 21 0° · 17% leak 10° · 26% leak 20° · 32% leak 30° · 34% leak T73 Round Brilliant in spinel · Leakage 6 0° · 0% leak 10° · 15% leak 20° · 30% leak 30° · 32% leak Each frame is the Light Return map with the stone tilted away from you by the angle shown; gray is leakage. The share under each frame is that picture's own. **What helps:** [the DG-M5](https://digitalgemology.com/gemstones/spinel/dg-m5) reads 0% for leakage in spinel, the best of the cuts we score there. [Try it yourself: Tilt it yourself in the simulator's light map](https://digitalgemology.com/tools/3d_gemstone_simulator?cut=octagon&stone=spinel&ct=1.00&view=up&map=1&turn=0) ### Fewer facets, less sparkle **Scintillation 66** against 80 for the T73 Round Brilliant in spinel: 14 points lower Scintillation follows the number of facets the stone shows: 100 × (1 − e^(−facets / 45)). The Octagon Cut has 49 facets and the T73 73, so 66 against 80. It depends on the geometry alone, so it is the same in every stone. *Facet diagram of the Octagon Cut: its crown, seen face-up, 49 facets in all* Octagon Cut · 49 facets · Scintillation 66 *Facet diagram of the T73 Round Brilliant: its crown, seen face-up, 73 facets in all* T73 Round Brilliant · 73 facets · Scintillation 80 Scintillation follows the number of facets: more facets, more and smaller flashes as the stone moves. Compare the facet counts under the drawings; each drawing shows only the crown. Drawn from each cut's own model. **What helps:** [the DG-Miliano](https://digitalgemology.com/gemstones/spinel/dg-miliano) reads 98 for scintillation in spinel, the best of the cuts we score there. [Try it yourself: Compare the Scintillation spoke in the Cut Performance Wheel](https://digitalgemology.com/tools/cut-performance-wheel?cut=octagon&stone=spinel) ## Our verdict on the Octagon Cut in spinel Against the T73 in spinel and the same cut in diamond, no figure of the Octagon Cut is ahead by our thresholds. The catch: it shows less fire than in diamond, returns less light than the T73, leaks more light than the T73 and has fewer facets to sparkle with. If that catch matters more to you, [the DG-Elongated Hexagon](https://digitalgemology.com/gemstones/spinel/dg-elongated-hexagon) reads 80 for fire in spinel, the best of the cuts we score there. Every figure on this page is from our light census, traced at one refractive index: a simulation of the geometry, not a grading report. The 3D files and cutting files below are the same Octagon Cut model we traced. ## Octagon Cut Spinel: questions ### Is the Octagon a good cut for spinel? In our census the Octagon Cut returns 78 of 100 for brilliance and 41 for fire in spinel, against 94 and 47 for the T73 in the same stone. Against that round and against the same cut in diamond it has 0 pros and 4 cons by our thresholds; the summary above names them. ### What pavilion angle does spinel need? At least 36.1° for a round design: spinel's critical angle is 35.7°, from the low end of its refractive index range, and head-on light has to reflect off both sides of the pavilion and leave through the table. The Octagon Cut's pavilion mains sit at 29.3°. ### How much does a Octagon weigh in spinel? At 6.5 mm across, 1.27 ct in spinel, against 1.23 ct for the same cut in diamond (+3%): spinel's specific gravity is 3.62, diamond's 3.52. ### Can I see the Octagon in spinel in 3D? Yes: the 3D Gemstone Simulator renders it live, with spinel's own refractive index and dispersion. Spinel is traced by the same light census as the other gemstones, with its refractive index and fire spread derived from published values, so its scores are on the same scale; the pictures here are our Light Return tracer, and the Cut Performance Wheel sets it against any other cut and stone. ## Get the Octagon Cut files *6 formats* *Millimeters* *Watertight* ### Octagon Cut 3D files The Octagon Cut as exact geometry: 49 planar facets, real-world scale, ready for CAD, rendering, printing and casting. - **STEP** Exact faceted solid for CAD - **3DM** Rhino, closed mesh on its own layer - **STL** Watertight mesh for printing and casting - **OBJ** Mesh for modeling and rendering - **FBX** Blender, 3ds Max, Maya, Cinema 4D, Unity, Unreal - **GLB** Web, AR and quick previews €9.99 · [Personal use](https://digitalgemology.com/licence) [Buy the 3D files](https://digitalgemology.lemonsqueezy.com/checkout/buy/26baac35-16bd-4143-97ba-9eeb7fd4da11) *GemCad* *96 index* ### Octagon Cut cutting files The Octagon Cut as a faceting design: 41 + 8 girdles = 49 facets, 96 index, 8-fold mirror-image symmetry, ready for the machine. - **.asc** GemCad design file, 96 index gear - **.csv** Tier table: angles, indices and cutting order - **Diagram** Printable faceting diagram €6.99 · [Personal use](https://digitalgemology.com/licence) [Buy the cutting files](https://digitalgemology.lemonsqueezy.com/checkout/buy/390b595d-894b-4e34-be35-57f44d569591) Save with a bundle: the Octagon Cut cutting files are also in [Faceter's Classics](https://digitalgemology.com/cuts/#bundles), 10 cuts for €22.49 until 1 December, then €29.99 (€69.90 as single files). [Buy the bundle](https://digitalgemology.lemonsqueezy.com/checkout/buy/9170b0c0-5357-490e-b6ef-3ce4f7fcd282?checkout%5Bdiscount_code%5D=BUNDLE25). Not sure the files open in your software? [Get the cube free](https://digitalgemology.lemonsqueezy.com/checkout/buy/2d1ab55f-2bd6-4897-a4cd-f3320f54e99e): the same six formats, for your email address, used only to send them ([privacy](https://digitalgemology.com/privacy#free-sample)). Questions before you buy? [Email us](mailto:support@digitalgemology.com). ## More gemstones and cuts [Spinel: optical properties and the top ten cuts](https://digitalgemology.com/gemstones/spinel) · [The Octagon Cut: specifications, model and files](https://digitalgemology.com/cuts/octagon) ### Other cuts in spinel - [T73 Round Brilliant](https://digitalgemology.com/gemstones/spinel/t73-round-brilliant) - [T105 Round Brilliant](https://digitalgemology.com/gemstones/spinel/t105-round-brilliant) - [DG-Vitruvio](https://digitalgemology.com/gemstones/spinel/dg-vitruvio) - [DG-Anteros](https://digitalgemology.com/gemstones/spinel/dg-anteros) - [DG-Broken Hearts and Arrows](https://digitalgemology.com/gemstones/spinel/dg-broken-hearts-and-arrows) - [Baguette Cut](https://digitalgemology.com/gemstones/spinel/baguette) - [Emerald Cut](https://digitalgemology.com/gemstones/spinel/emerald-cut) - [Marquise Cut](https://digitalgemology.com/gemstones/spinel/marquise) - [Oval Cut](https://digitalgemology.com/gemstones/spinel/oval) - [Pear Cut](https://digitalgemology.com/gemstones/spinel/pear) - [Radiant Cut](https://digitalgemology.com/gemstones/spinel/radiant) - [Rectangle Cut](https://digitalgemology.com/gemstones/spinel/rectangle) - [Teardrop Cut](https://digitalgemology.com/gemstones/spinel/teardrop) - [Asscher Cut](https://digitalgemology.com/gemstones/spinel/asscher) - [Cushion Cut](https://digitalgemology.com/gemstones/spinel/cushion) - [Emerald Square Cut](https://digitalgemology.com/gemstones/spinel/emerald-square) - [Princess Cut](https://digitalgemology.com/gemstones/spinel/princess) - [Radiant Square Cut](https://digitalgemology.com/gemstones/spinel/radiant-square) - [Square Cut](https://digitalgemology.com/gemstones/spinel/square) - [Triangle Cut](https://digitalgemology.com/gemstones/spinel/triangle) - [Trilliant Cut](https://digitalgemology.com/gemstones/spinel/trilliant) - [Trillion Cut](https://digitalgemology.com/gemstones/spinel/trillion) - [Hexagon Cut](https://digitalgemology.com/gemstones/spinel/hexagon) - [Portuguese Bird's Nest Cut](https://digitalgemology.com/gemstones/spinel/portuguese-birds-nest) - [Old Single Cut](https://digitalgemology.com/gemstones/spinel/old-single) - [Mazarin Cut](https://digitalgemology.com/gemstones/spinel/mazarin) - [Peruzzi Cut](https://digitalgemology.com/gemstones/spinel/peruzzi) - [Old Mine Cut](https://digitalgemology.com/gemstones/spinel/old-mine) - [Old European Cut](https://digitalgemology.com/gemstones/spinel/old-european) - [T57 Round Brilliant](https://digitalgemology.com/gemstones/spinel/t57-round-brilliant) - [DG-M5](https://digitalgemology.com/gemstones/spinel/dg-m5) - [DG-Miliano](https://digitalgemology.com/gemstones/spinel/dg-miliano) - [DG-Rafaello](https://digitalgemology.com/gemstones/spinel/dg-rafaello) - [DG-Bowtie](https://digitalgemology.com/gemstones/spinel/dg-bowtie) - [DG-Elongated Hexagon](https://digitalgemology.com/gemstones/spinel/dg-elongated-hexagon) - [DG-Octagonal Kite](https://digitalgemology.com/gemstones/spinel/dg-octagonal-kite) ### The Octagon Cut in other gemstones - [Moissanite](https://digitalgemology.com/gemstones/moissanite/octagon) - [Diamond](https://digitalgemology.com/gemstones/diamond/octagon) - [Cubic Zirconia](https://digitalgemology.com/gemstones/cubic-zirconia/octagon) - [Ruby](https://digitalgemology.com/gemstones/ruby/octagon) - [Sapphire](https://digitalgemology.com/gemstones/sapphire/octagon) - [Emerald](https://digitalgemology.com/gemstones/emerald/octagon) - [Amethyst](https://digitalgemology.com/gemstones/amethyst/octagon) - [Topaz](https://digitalgemology.com/gemstones/topaz/octagon) - [Demantoid](https://digitalgemology.com/gemstones/demantoid/octagon) - [Zircon](https://digitalgemology.com/gemstones/zircon/octagon) - [Tourmaline](https://digitalgemology.com/gemstones/tourmaline/octagon) - [Peridot](https://digitalgemology.com/gemstones/peridot/octagon) - [Tanzanite](https://digitalgemology.com/gemstones/tanzanite/octagon) - [YAG](https://digitalgemology.com/gemstones/yag/octagon) - [Pyrope](https://digitalgemology.com/gemstones/pyrope/octagon) - [Almandine](https://digitalgemology.com/gemstones/almandine/octagon) - [Spessartine](https://digitalgemology.com/gemstones/spessartine/octagon) - [Grossular](https://digitalgemology.com/gemstones/grossular/octagon) - [Chrysoberyl](https://digitalgemology.com/gemstones/chrysoberyl/octagon) - [Kunzite](https://digitalgemology.com/gemstones/kunzite/octagon) - [Iolite](https://digitalgemology.com/gemstones/iolite/octagon) - [Morganite](https://digitalgemology.com/gemstones/morganite/octagon) - [GGG](https://digitalgemology.com/gemstones/ggg/octagon) - [Strontium titanate](https://digitalgemology.com/gemstones/strontium-titanate/octagon) - [Synthetic rutile](https://digitalgemology.com/gemstones/synthetic-rutile/octagon)