Brilliance 65 · Fire 53 · Scintillation 90 · in our census, where the T73 scores 72 · 58 · 80 in iolite
How the T105 Round Brilliant handles light in iolite, measured: brilliance 65, fire 53 and 35% 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 T105 Round Brilliant in iolite live.
The T105 Round Brilliant has 105 facets and a total depth of 59.3% of its width; its pavilion mains sit at 40.8°. In iolite, the census returns 65 of 100 for brilliance and 53 for fire, against 72 and 58 for the T73, our benchmark, in the same stone; the same T105 Round Brilliant in diamond reads 95 and 77.
Compared with the T73 Round Brilliant in iolite and with the T105 Round Brilliant 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 T105 Round Brilliant 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.
0.68 ct at 6.5 mm, against 0.92 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 0.68 ct here and 0.92 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the T105 Round Brilliant page
Scintillation 90 against 80 for the T73 Round Brilliant in iolite: 10 points higher
Scintillation follows the number of facets the stone shows: 100 × (1 − e^(−facets / 45)). The T105 Round Brilliant has 105 facets and the T73 73, so 90 against 80. It depends on the geometry alone, so it is the same in every stone.
T105 Round Brilliant · 105 facets · Scintillation 90
T73 Round Brilliant · 73 facets · Scintillation 80
Try it yourself: Compare the Scintillation spoke in the Cut Performance Wheel
Face-up size 106 against 100 for the T73 Round Brilliant in iolite (T73 = 100): 6% more face-up area for the same weight
Face-up size is the outline's area per carat, measured from each model (T73 = 100): 106 for the T105 Round Brilliant 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.
T105 Round Brilliant · 7.39 mm at 1.00 ct · Face-up size 106
T73 Round Brilliant · 7.15 mm at 1.00 ct · Face-up size 100
Try it yourself: See its size at 1.00 ct in iolite in the 3D Gemstone Simulator
Leakage 35 against 1 for the T105 Round Brilliant in diamond: 34 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 T105 Round Brilliant leaks 35% in iolite against 1% for the same cut in diamond. Grey in the maps below is leaking light; the share under each picture is that picture's own.
T105 Round Brilliant in iolite · Leakage 35
T105 Round Brilliant in diamond · Leakage 1
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
Brilliance 65 against 95 for the T105 Round Brilliant in diamond: 30 points lower
The same cut in diamond returns 95. Iolite bends light less than diamond: its critical angle is 40.4°, diamond's 24.4°, so a pavilion facet has to be steeper before light reflects off it rather than passing out. The T105 Round Brilliant's pavilion returns less of iolite's light than of diamond's, as the two maps show.
T105 Round Brilliant in iolite · Brilliance 65
T105 Round Brilliant in diamond · Brilliance 95
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
Fire 53 against 77 for the T105 Round Brilliant in diamond: 24 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 (0.015 in iolite) and counts a ray as fire when the two directions differ by more than 0.6°. The T105 Round Brilliant reads 53 against 77 for the same cut in diamond. 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 T105 Round Brilliant in iolite under the sunny-day lighting.
What helps: the Oval Cut reads 82 for fire in iolite, the best of the cuts we score there.
Pick the T105 Round Brilliant in iolite for this: it weighs less than in diamond, has more facets to sparkle with and faces up larger for its weight. The catch: it leaks more than in diamond, returns less light than in diamond and shows less fire than in diamond.
If that catch matters more to you, the DG-M5 reads 15% for leakage 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 T105 Round Brilliant model we traced.
In our census the T105 Round Brilliant returns 65 of 100 for brilliance and 53 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 3 pros and 3 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 T105 Round Brilliant's pavilion mains sit at 40.8°.
At 6.5 mm across, 0.68 ct in iolite, against 0.92 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 T105 Round Brilliant Cut as exact geometry: 105 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The T105 Round Brilliant Cut as a faceting design: 105 facets, 96 index, 8-fold mirror-image 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 T105 Round Brilliant: specifications, model and files