Octagon Cut GGG: Pros, Cons and Scores

GGG has a refractive index of 1.970 and a dispersion of 0.045. Its critical angle, from the low end of that range, is 30.5°, so a round design needs a pavilion of at least 37.4° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.970, the midpoint of the published range, and a fire spread of 0.039, 0.86 of the published dispersion. Every picture here is our Light Return tracer at 1.970, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the Octagon Cut in GGG 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 GGG, the census returns 75 of 100 for brilliance and 69 for fire, against 98 and 76 for the T73, our benchmark, in the same stone; the same Octagon Cut in diamond reads 71 and 76.

Is GGG good in an Octagon? Pros and cons, measured

Pros

Holds its light better in GGG than in diamond 100 vs 97

Cons

Heavier than the same cut in diamond 2.47 vs 1.23 ct
Returns less light than the T73 in GGG 75 vs 98
Leaks more light than the T73 in GGG 21 vs 2
Fewer facets, less sparkle 66 vs 80

Compared with the T73 Round Brilliant in GGG 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.

All eight scores against the T73

At a glance: the Octagon Cut in GGG (white) against the T73 Round Brilliant in GGG (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 ggg: ahead on 1, level on 2, behind on 5.

Global80, the comparison 90, difference −10
Brilliance75, the comparison 98, difference −23
Fire69, the comparison 76, difference −7
Scintillation66, the comparison 80, difference −14
Spread100, the comparison 100, difference 0
Symmetry100, the comparison 100, difference 0
Shape-norm99, the comparison 105, difference −6
Tilt100, the comparison 82, difference +18

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; these are the same census figures as our Cut Performance Index.

What the Octagon Cut gains in GGG

Holds its light better in GGG than in diamond

Tilt 100 against 97 for the Octagon Cut in diamond: 3 points higher

Tilt is how much of its face-up brilliance a stone keeps when the census tilts it 20° about its long axis, with 2,200 rays. The Octagon Cut keeps 100 against 97 for the same cut in diamond. The tilt strips below show the maps at 0°, 10°, 20° and 30°, with each picture's own leak share under it.

Octagon cut in ggg tilted 0 degrees: Light Return map, 26% of the face leaking — Tilt is how much of its own face-up light a stone keeps when tilted. 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.

Octagon cut in ggg tilted 10 degrees: Light Return map, 16% of the face leaking — Tilt is how much of its own face-up light a stone keeps when tilted. 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.

Try it yourself: Compare the Tilt spoke in the Cut Performance Wheel

What the Octagon Cut gives up in GGG

Heavier than the same cut in diamond

2.47 ct at 6.5 mm, against 1.23 ct for the same cut in diamond (+100%)

Weight follows specific gravity: 7.05 for GGG against 3.52 for diamond, so the same model at the same size weighs 2.47 ct here and 1.23 ct in diamond, 6.5 mm across. Nothing about the light changes with it.

A Octagon Cut 6.5 mm across, weighed from the model's measured volume and each stone's specific gravity.

Try it yourself: Weights in every gemstone on the Octagon Cut page

Returns less light than the T73 in GGG

Brilliance 75 against 98 for the T73 Round Brilliant in GGG: 23 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 GGG, whose critical angle is 30.5°, the Octagon Cut returns 75 of 100 and the T73 98. 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 ggg face-up Light Return map: red, green and blue show where the light returns from, gray shows where it leaks (Brilliance 75) — 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.

T73 round brilliant in ggg face-up Light Return map: red, green and blue show where the light returns from, gray shows where it leaks (Brilliance 98) — 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-Broken Hearts and Arrows reads 98 for brilliance in GGG, the best of the cuts we score there.

Try it yourself: Open this Light Return map in the simulator

Leaks more light than the T73 in GGG

Leakage 21 against 2 for the T73 Round Brilliant in GGG: 19 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 GGG against 2% for the T73 in GGG. Grey in the maps below is leaking light; the share under each picture is that picture's own.

What helps: the DG-Broken Hearts and Arrows reads 2% for leakage in GGG, the best of the cuts we score there.

Try it yourself: Tilt it yourself in the simulator's light map

Fewer facets, less sparkle

Scintillation 66 against 80 for the T73 Round Brilliant in GGG: 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.

Octagon Cut · 49 facets · Scintillation 66

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 reads 98 for scintillation in GGG, the best of the cuts we score there.

Try it yourself: Compare the Scintillation spoke in the Cut Performance Wheel

Our verdict on the Octagon Cut in GGG

Pick the Octagon Cut in GGG for this: it holds its light better than in diamond when tilted. The catch: it weighs more 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-Broken Hearts and Arrows reads 98 for brilliance in GGG, 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 GGG: questions

Is the Octagon a good cut for GGG?

In our census the Octagon Cut returns 75 of 100 for brilliance and 69 for fire in GGG, against 98 and 76 for the T73 in the same stone. Against that round and against the same cut in diamond it has 1 pro and 4 cons by our thresholds; the summary above names them.

What pavilion angle does GGG need?

At least 37.4° for a round design: GGG's critical angle is 30.5°, 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 GGG?

At 6.5 mm across, 2.47 ct in GGG, against 1.23 ct for the same cut in diamond (+100%): GGG's specific gravity is 7.05, diamond's 3.52.

Can I see the Octagon in GGG in 3D?

Yes: the 3D Gemstone Simulator renders it live, with GGG's own refractive index and dispersion. GGG 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 formatsMillimetersWatertight

Octagon Cut 3D files

The Octagon Cut as exact geometry: 49 planar facets, real-world scale, ready for CAD, rendering, printing and casting.

STEPExact faceted solid for CAD
3DMRhino, closed mesh on its own layer
STLWatertight mesh for printing and casting
OBJMesh for modeling and rendering
FBXBlender, 3ds Max, Maya, Cinema 4D, Unity, Unreal
GLBWeb, AR and quick previews

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GemCad96 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.

.ascGemCad design file, 96 index gear
.csvTier table: angles, indices and cutting order
DiagramPrintable faceting diagram

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Save with a bundle: the Octagon Cut cutting files are also in Faceter's Classics, 10 cuts for €22.49 until 1 December, then €29.99 (€69.90 as single files). Buy the bundle.

Not sure the files open in your software? Get the cube free: the same six formats, for your email address, used only to send them (privacy).

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More gemstones and cuts

GGG: optical properties and the top ten cuts · The Octagon Cut: specifications, model and files

Octagon Cut GGG: holds its light better in GGG than in diamond