Brilliance 28 · Fire 72 · Scintillation 34 · in our census, where the T73 scores 72 · 58 · 80 in iolite
How the Triangle Cut handles light in iolite, measured: brilliance 28, fire 72 and 43% leakage, set against the T73 in the same stone and against the same cut in diamond.
Iolite has a refractive index of 1.542 to 1.551 and a dispersion of 0.017. Its critical angle, from the low end of that range, is 40.4°, so a round design needs a pavilion of at least 40.4° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.547, the midpoint of the published range, and a fire spread of 0.015, 0.86 of the published dispersion. Every picture here is our Light Return tracer at 1.542, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the Triangle Cut in iolite live.
The Triangle Cut has 19 facets, a table of 60.0% of its width and a total depth of 55.4% of its width; its pavilion mains sit at 41.8°. In iolite, the census returns 28 of 100 for brilliance and 72 for fire, against 72 and 58 for the T73, our benchmark, in the same stone; the same Triangle Cut in diamond reads 53 and 86.
Compared with the T73 Round Brilliant in iolite and with the Triangle 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 Triangle Cut in iolite (white) against the T73 Round Brilliant in iolite (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 iolite: ahead on 3, level on 1, behind on 4.
0.59 ct at 6.5 mm, against 0.79 ct for the same cut in diamond (−26%)
Weight follows specific gravity: 2.61 for iolite against 3.52 for diamond, so the same model at the same size weighs 0.59 ct here and 0.79 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the Triangle Cut page
Fire 72 against 58 for the T73 Round Brilliant in iolite: 14 points higher
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 (0.015 in iolite) and counts a ray as fire when the two directions differ by more than 0.6°. The Triangle Cut reads 72 against 58 for the T73 in iolite. The figure is the census's; the 3D Gemstone Simulator renders iolite'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 Triangle Cut in iolite under the sunny-day lighting.
Scintillation 34 against 80 for the T73 Round Brilliant in iolite: 46 points lower
Scintillation follows the number of facets the stone shows: 100 × (1 − e^(−facets / 45)). The Triangle Cut has 19 facets and the T73 73, so 34 against 80. It depends on the geometry alone, so it is the same in every stone.
Triangle Cut · 19 facets · Scintillation 34
T73 Round Brilliant · 73 facets · Scintillation 80
What helps: the DG-Miliano reads 98 for scintillation in iolite, the best of the cuts we score there.
Try it yourself: Compare the Scintillation spoke in the Cut Performance Wheel
Brilliance 28 against 72 for the T73 Round Brilliant in iolite: 44 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 iolite, whose critical angle is 40.4°, the Triangle Cut returns 28 of 100 and the T73 72. The Triangle Cut's pavilion mains sit at 41.8°, the T73's at 40.8°. The maps below show where each stone's light returns from and where it leaks.
Triangle Cut in iolite · Brilliance 28
T73 Round Brilliant in iolite · Brilliance 72
What helps: the DG-M5 reads 85 for brilliance in iolite, the best of the cuts we score there.
Try it yourself: Open this Light Return map in the simulator
Leakage 43 against 25 for the Triangle Cut in diamond: 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 Triangle Cut leaks 43% in iolite against 25% for the same cut in diamond. Grey in the maps below is leaking light; the share under each picture is that picture's own.
Triangle Cut in iolite · Leakage 43
Triangle Cut in diamond · Leakage 25
What helps: the DG-M5 reads 15% for leakage in iolite, the best of the cuts we score there.
Try it yourself: Tilt it yourself in the simulator's light map
Face-up size 89 against 100 for the T73 Round Brilliant in iolite (T73 = 100): 11% less face-up area for the same weight
Face-up size is the outline's area per carat, measured from each model (T73 = 100): 89 for the Triangle Cut against 100 for the T73. The same volume spread over a longer or shallower outline shows more face for the weight; it is the same in every stone, because specific gravity scales every cut alike.
Triangle Cut · 8.98 x 7.77 mm at 1.00 ct · Face-up size 89
T73 Round Brilliant · 7.15 mm at 1.00 ct · Face-up size 100
What helps: the Marquise Cut reads 121 for face-up size in iolite, the best of the cuts we score there.
Try it yourself: See its size at 1.00 ct in iolite in the 3D Gemstone Simulator
Pick the Triangle Cut in iolite for this: it weighs less than in diamond and shows more fire than the T73. The catch: it has fewer facets to sparkle with, returns less light than the T73, leaks more than in diamond and faces up smaller for its weight.
If that catch matters more to you, the DG-Miliano reads 98 for scintillation in iolite, 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 Triangle Cut model we traced.
In our census the Triangle Cut returns 28 of 100 for brilliance and 72 for fire in iolite, against 72 and 58 for the T73 in the same stone. Against that round and against the same cut in diamond it has 2 pros and 4 cons by our thresholds; the summary above names them.
At least 40.4° for a round design: iolite's critical angle is 40.4°, 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 Triangle Cut's pavilion mains sit at 41.8°.
At 6.5 mm across, 0.59 ct in iolite, against 0.79 ct for the same cut in diamond (-26%): iolite's specific gravity is 2.61, diamond's 3.52.
Yes: the 3D Gemstone Simulator renders it live, with iolite's own refractive index and dispersion. Iolite 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 Triangle Cut as exact geometry: 19 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The Triangle Cut as a faceting design: 16 + 3 girdles = 19 facets, 96 index, 3-fold symmetry, ready for the machine.
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.
Iolite: optical properties and the top ten cuts · The Triangle Cut: specifications, model and files