Octagon Cut Demantoid: what our light census measures
Brilliance 77 · Fire 71 · Scintillation 66 · in our census, where the T73 scores 97 · 84 · 80 in demantoid
How the Octagon Cut handles light in demantoid, measured: brilliance 77, fire 71 and 21% leakage, set against the T73 in the same stone and against the same cut in diamond.
Light Return map of the Octagon Cut in demantoid, face-up, from our own tracer at refractive index 1.881. Red, green and blue return light to the eye; gray leaks.
Octagon Cut Demantoid
Demantoid has a refractive index of 1.881 to 1.888 and a dispersion of 0.057. Its critical angle, from the low end of that range, is 32.1°, so a round design needs a pavilion of at least 37.0° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.885, the midpoint of the published range, and a fire spread of 0.049, 0.86 of the published dispersion. Every picture here is our Light Return tracer at 1.881, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the Octagon Cut in demantoid 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 demantoid, the census returns 77 of 100 for brilliance and 71 for fire, against 97 and 84 for the T73, our benchmark, in the same stone; the same Octagon Cut in diamond reads 71 and 76.
Is demantoid good in an Octagon? Pros and cons, measured
Compared with the T73 Round Brilliant in demantoid 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 demantoid (white) against the T73 Round Brilliant in demantoid (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 demantoid: ahead on 1, level on 2, behind on 5.
Global80, the comparison 90, difference −10
Brilliance77, the comparison 97, difference −20
Fire71, the comparison 84, difference −13
Scintillation66, the comparison 80, difference −14
Spread100, the comparison 100, difference 0
Symmetry100, the comparison 100, difference 0
Shape-norm101, the comparison 104, difference −3
Tilt96, the comparison 79, difference +17
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 demantoid
Against the T73 in demantoid 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; Demantoid ranks every cut we score in demantoid.
What the Octagon Cut gives up in demantoid
Returns less light than the T73 in demantoid
Brilliance 77 against 97 for the T73 Round Brilliant in demantoid: 20 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 demantoid, whose critical angle is 32.1°, the Octagon Cut returns 77 of 100 and the T73 97. 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 demantoid · Brilliance 77
T73 Round Brilliant in demantoid · Brilliance 97
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.
Leakage 21 against 3 for the T73 Round Brilliant in demantoid: 18 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 demantoid against 3% for the T73 in demantoid. Grey in the maps below is leaking light; the share under each picture is that picture's own.
Octagon Cut in demantoid · Leakage 21
0° · 27% leak
10° · 18% leak
20° · 23% leak
30° · 32% leak
T73 Round Brilliant in demantoid · Leakage 3
0° · 1% leak
10° · 6% leak
20° · 17% leak
30° · 29% 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.
Scintillation 66 against 80 for the T73 Round Brilliant in demantoid: 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.
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 demantoid, the best of the cuts we score there.
Fire 71 against 84 for the T73 Round Brilliant in demantoid: 13 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 (0.049 in demantoid) and counts a ray as fire when the two directions differ by more than 0.6°. The Octagon Cut reads 71 against 84 for the T73 in demantoid. The figure is the census's; the 3D Gemstone Simulator renders demantoid'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 demantoid under the sunny-day lighting.
What helps:the DG-Elongated Hexagon reads 97 for fire in demantoid, the best of the cuts we score there.
Against the T73 in demantoid and the same cut in diamond, no figure of the Octagon Cut is ahead by our thresholds. The catch: it returns less light than the T73, leaks more light than the T73, has fewer facets to sparkle with and shows less fire than the T73.
If that catch matters more to you, the DG-Broken Hearts and Arrows reads 97 for brilliance in demantoid, 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 Demantoid: questions
Is the Octagon a good cut for demantoid?
In our census the Octagon Cut returns 77 of 100 for brilliance and 71 for fire in demantoid, against 97 and 84 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 demantoid need?
At least 37.0° for a round design: demantoid's critical angle is 32.1°, 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°.
How much does a Octagon weigh in demantoid?
At 6.5 mm across, 1.35 ct in demantoid, against 1.23 ct for the same cut in diamond (+9%): demantoid's specific gravity is 3.85, diamond's 3.52.
Can I see the Octagon in demantoid in 3D?
Yes: the 3D Gemstone Simulator renders it live, with demantoid's own refractive index and dispersion. Demantoid 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.
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).
