T105 Round Brilliant Spinel: more facets, more sparkle
Brilliance 89 · Fire 47 · Scintillation 90 · in our census, where the T73 scores 94 · 47 · 80 in spinel
How the T105 Round Brilliant handles light in spinel, measured: brilliance 89, fire 47 and 11% leakage, set against the T73 in the same stone and against the same cut in diamond.
Light Return map of the T105 Round Brilliant in spinel, face-up, from our own tracer at refractive index 1.712. Red, green and blue return light to the eye; gray leaks.
T105 Round Brilliant Spinel
Spinel has a refractive index of 1.712 to 1.747 and a dispersion of 0.020. Its critical angle, from the low end of that range, is 35.7°, so a round design needs a pavilion of at least 36.1° for head-on light to reflect off both sides of the pavilion and leave through the table. We trace it with one index, 1.730, the midpoint of the published range, and a fire spread of 0.017, 0.86 of the published dispersion. Every picture here is our Light Return tracer at 1.712, and every figure is from the same light census that scores the other gemstones; the 3D Gemstone Simulator shows the T105 Round Brilliant in spinel 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 spinel, the census returns 89 of 100 for brilliance and 47 for fire, against 94 and 47 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 spinel 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.
All eight scores against the T73
At a glance: the T105 Round Brilliant in spinel (white) against the T73 Round Brilliant in spinel (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 spinel: ahead on 3, level on 2, behind on 3.
Global80, the comparison 81, difference −1
Brilliance89, the comparison 94, difference −5
Fire47, the comparison 47, difference 0
Scintillation90, the comparison 80, difference +10
Spread101, the comparison 100, difference +1
Symmetry100, the comparison 100, difference 0
Shape-norm96, the comparison 101, difference −5
Tilt78, the comparison 75, difference +3
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 T105 Round Brilliant gains in spinel
More facets, more sparkle
Scintillation 90 against 80 for the T73 Round Brilliant in spinel: 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.
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.
Face-up size 106 against 100 for the T73 Round Brilliant in spinel (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 · 6.62 mm at 1.00 ct · Face-up size 106
T73 Round Brilliant · 6.42 mm at 1.00 ct · Face-up size 100
Both stones weigh 1.00 ct in spinel and are drawn to the same scale in millimeters, from each cut's own model. Face-up size is the outline's area per carat, with the T73 at 100: a deeper stone, or a rounder outline, spreads less area for its weight.
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 (0.017 in spinel) 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 spinel'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 spinel under the sunny-day lighting.
What helps:the DG-Elongated Hexagon reads 80 for fire in spinel, the best of the cuts we score there.
Leakage 11 against 1 for the T105 Round Brilliant in diamond: 10 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 11% in spinel 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 spinel · Leakage 11
0° · 13% leak
10° · 16% leak
20° · 30% leak
30° · 31% leak
T105 Round Brilliant in diamond · Leakage 1
0° · 2% leak
10° · 2% leak
20° · 11% leak
30° · 14% 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.
What helps:the DG-M5 reads 0% for leakage in spinel, the best of the cuts we score there.
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 spinel · Tilt 78
0° · 13% leak
10° · 16% leak
20° · 30% leak
30° · 31% leak
T105 Round Brilliant in diamond · Tilt 87
0° · 2% leak
10° · 2% leak
20° · 11% leak
30° · 14% leak
Tilt is how much of its own face-up light a stone keeps when tilted. 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.
What helps:the DG-Miliano reads 82 for tilt in spinel, the best of the cuts we score there.
Brilliance 89 against 95 for the T105 Round Brilliant in diamond: 6 points lower
The same cut in diamond returns 95. Spinel bends light less than diamond: its critical angle is 35.7°, 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 spinel's light than of diamond's, as the two maps show.
T105 Round Brilliant in spinel · Brilliance 89
T105 Round Brilliant in diamond · Brilliance 95
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.
What helps:the DG-M5 reads 100 for brilliance in spinel, the best of the cuts we score there.
Pick the T105 Round Brilliant in spinel for this: it 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, loses more light when tilted than in diamond and returns less light than in diamond.
If that catch matters more to you, the DG-Elongated Hexagon reads 80 for fire in spinel, 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.
T105 Round Brilliant Spinel: questions
Is the T105 a good cut for spinel?
In our census the T105 Round Brilliant returns 89 of 100 for brilliance and 47 for fire in spinel, against 94 and 47 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.
What pavilion angle does spinel need?
At least 36.1° for a round design: spinel's critical angle is 35.7°, 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°.
How much does a T105 weigh in spinel?
At 6.5 mm across, 0.94 ct in spinel, against 0.92 ct for the same cut in diamond (+3%): spinel's specific gravity is 3.62, diamond's 3.52.
Can I see the T105 in spinel in 3D?
Yes: the 3D Gemstone Simulator renders it live, with spinel's own refractive index and dispersion. Spinel 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 T105 Round Brilliant Cut files
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T105 Round Brilliant Cut 3D files
The T105 Round Brilliant Cut as exact geometry: 105 planar facets, real-world scale, ready for CAD,
rendering, printing and casting.
