Brilliance 78 · Fire 51 · Scintillation 66 · in our census, where the T73 scores 96 · 54 · 80 in YAG
How the Octagon Cut handles light in YAG, measured: brilliance 78, fire 51 and 20% leakage, set against the T73 in the same stone and against the same cut in diamond.
YAG has a refractive index of 1.833 and a dispersion of 0.028. Its critical angle, from the low end of that range, is 33.1°, so a round design needs a pavilion of at least 36.7° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.833, the midpoint of the published range, and a fire spread of 0.024, 0.86 of the published dispersion. Every picture here is our Light Return tracer at 1.833, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the Octagon Cut in YAG 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 YAG, the census returns 78 of 100 for brilliance and 51 for fire, against 96 and 54 for the T73, our benchmark, in the same stone; the same Octagon Cut in diamond reads 71 and 76.
No figure ahead by our thresholds
Compared with the T73 Round Brilliant in YAG 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.
At a glance: the Octagon Cut in YAG (white) against the T73 Round Brilliant in YAG (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 yag: ahead on 1, level on 3, behind on 4.
Against the T73 in YAG 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; YAG ranks every cut we score in YAG.
1.59 ct at 6.5 mm, against 1.23 ct for the same cut in diamond (+29%)
Weight follows specific gravity: 4.55 for YAG against 3.52 for diamond, so the same model at the same size weighs 1.59 ct here and 1.23 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the Octagon Cut page
Fire 51 against 76 for the Octagon Cut in diamond: 25 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 (0.024 in YAG) and counts a ray as fire when the two directions differ by more than 0.6°. The Octagon Cut reads 51 against 76 for the same cut in diamond. The figure is the census's; the 3D Gemstone Simulator renders YAG'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: the Octagon Cut in YAG under the sunny-day lighting.
What helps: the DG-Elongated Hexagon reads 93 for fire in YAG, the best of the cuts we score there.
Brilliance 78 against 96 for the T73 Round Brilliant in YAG: 18 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 YAG, whose critical angle is 33.1°, the Octagon Cut returns 78 of 100 and the T73 96. 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 YAG · Brilliance 78
T73 Round Brilliant in YAG · Brilliance 96
What helps: the DG-M5 reads 97 for brilliance in YAG, the best of the cuts we score there.
Try it yourself: Open this Light Return map in the simulator
Leakage 20 against 4 for the T73 Round Brilliant in YAG: 16 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 20% in YAG against 4% for the T73 in YAG. Grey in the maps below is leaking light; the share under each picture is that picture's own.
Octagon Cut in YAG · Leakage 20
T73 Round Brilliant in YAG · Leakage 4
What helps: the DG-M5 reads 3% for leakage in YAG, the best of the cuts we score there.
Try it yourself: Tilt it yourself in the simulator's light map
Against the T73 in YAG and the same cut in diamond, no figure of the Octagon Cut is ahead by our thresholds. The catch: it weighs more than in diamond, shows less fire than in diamond, returns less light than the T73 and leaks more light than the T73.
If that catch matters more to you, the DG-Elongated Hexagon reads 93 for fire in YAG, 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.
In our census the Octagon Cut returns 78 of 100 for brilliance and 51 for fire in YAG, against 96 and 54 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.
At least 36.7° for a round design: YAG's critical angle is 33.1°, 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°.
At 6.5 mm across, 1.59 ct in YAG, against 1.23 ct for the same cut in diamond (+29%): YAG's specific gravity is 4.55, diamond's 3.52.
Yes: the 3D Gemstone Simulator renders it live, with YAG's own refractive index and dispersion. YAG 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.
The Octagon Cut as exact geometry: 49 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The Octagon Cut as a faceting design: 41 + 8 girdles = 49 facets, 96 index, 8-fold mirror-image symmetry, ready for the machine.
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).
Questions before you buy? Email us.
YAG: optical properties and the top ten cuts · The Octagon Cut: specifications, model and files