Brilliance 79 · Fire 47 · Scintillation 90 · in our census, where the T73 scores 86 · 50 · 80 in tourmaline
How the T105 Round Brilliant handles light in tourmaline, measured: brilliance 79, fire 47 and 21% leakage, set against the T73 in the same stone and against the same cut in diamond.
Tourmaline has a refractive index of 1.603 to 1.655 and a dispersion of 0.017. Its critical angle, from the low end of that range, is 38.6°, so a round design needs a pavilion of at least 38.6° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.629, 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 1.603, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the T105 Round Brilliant in tourmaline live. Tourmaline is strongly doubly refractive (birefringence up to 0.040), so its back facets can look doubled. Our census traces one index (1.629), so the doubling is not in these figures.
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 tourmaline, the census returns 79 of 100 for brilliance and 47 for fire, against 86 and 50 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 tourmaline 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 tourmaline (white) against the T73 Round Brilliant in tourmaline (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 tourmaline: ahead on 3, level on 1, behind on 4.
0.82 ct at 6.5 mm, against 0.92 ct for the same cut in diamond (−11%)
Weight follows specific gravity: 3.13 for tourmaline against 3.52 for diamond, so the same model at the same size weighs 0.82 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 tourmaline: 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 tourmaline (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 · 6.96 mm at 1.00 ct · Face-up size 106
T73 Round Brilliant · 6.74 mm at 1.00 ct · Face-up size 100
Try it yourself: See its size at 1.00 ct in tourmaline in the 3D Gemstone Simulator
Fire 47 against 77 for the T105 Round Brilliant in diamond: 30 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 tourmaline) and counts a ray as fire when the two directions differ by more than 0.6°. The T105 Round Brilliant reads 47 against 77 for the same cut in diamond. The figure is the census's; the 3D Gemstone Simulator renders tourmaline'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 tourmaline under the sunny-day lighting.
What helps: the Oval Cut reads 80 for fire in tourmaline, the best of the cuts we score there.
Leakage 21 against 1 for the T105 Round Brilliant in diamond: 20 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 21% in tourmaline 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 tourmaline · Leakage 21
T105 Round Brilliant in diamond · Leakage 1
What helps: the DG-M5 reads 4% for leakage in tourmaline, the best of the cuts we score there.
Try it yourself: Tilt it yourself in the simulator's light map
Brilliance 79 against 95 for the T105 Round Brilliant in diamond: 16 points lower
The same cut in diamond returns 95. Tourmaline bends light less than diamond: its critical angle is 38.6°, 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 tourmaline's light than of diamond's, as the two maps show.
T105 Round Brilliant in tourmaline · Brilliance 79
T105 Round Brilliant in diamond · Brilliance 95
What helps: the DG-M5 reads 96 for brilliance in tourmaline, the best of the cuts we score there.
Try it yourself: Open this Light Return map in the simulator
Tilt 78 against 87 for the T105 Round Brilliant in diamond: 9 points lower
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 T105 Round Brilliant keeps 78 against 87 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.
T105 Round Brilliant in tourmaline · Tilt 78
T105 Round Brilliant in diamond · Tilt 87
What helps: the DG-Miliano reads 86 for tilt in tourmaline, the best of the cuts we score there.
Try it yourself: Compare the Tilt spoke in the Cut Performance Wheel
Pick the T105 Round Brilliant in tourmaline for this: it weighs less than in diamond, has more facets to sparkle with and faces up larger for its weight. The catch: it shows less fire than in diamond, leaks more than in diamond, returns less light than in diamond and loses more light when tilted than in diamond.
If that catch matters more to you, the Oval Cut reads 80 for fire in tourmaline, 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 79 of 100 for brilliance and 47 for fire in tourmaline, against 86 and 50 for the T73 in the same stone. Against that round and against the same cut in diamond it has 3 pros and 4 cons by our thresholds; the summary above names them.
At least 38.6° for a round design: tourmaline's critical angle is 38.6°, 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, 0.82 ct in tourmaline, against 0.92 ct for the same cut in diamond (-11%): tourmaline's specific gravity is 3.13, diamond's 3.52.
Yes: the 3D Gemstone Simulator renders it live, with tourmaline's own refractive index and dispersion. Tourmaline 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.
Tourmaline: optical properties and the top ten cuts · The T105 Round Brilliant: specifications, model and files