Brilliance 24 · Fire 60 · Scintillation 63 · in our census, where the T73 scores 72 · 58 · 80 in iolite
How the Rectangle Cut handles light in iolite, measured: brilliance 24, fire 60 and 72% 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 Rectangle Cut in iolite live.
The Rectangle Cut has 45 facets and a total depth of 62.0% of its width; its pavilion mains sit at 33.5°. In iolite, the census returns 24 of 100 for brilliance and 60 for fire, against 72 and 58 for the T73, our benchmark, in the same stone; the same Rectangle Cut in diamond reads 50 and 77.
Compared with the T73 Round Brilliant in iolite and with the Rectangle 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 Rectangle 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 2, level on 2, behind on 4.
1.56 ct at 6.5 mm, against 2.10 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 1.56 ct here and 2.10 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the Rectangle Cut page
Face-up size 121 against 100 for the T73 Round Brilliant in iolite (T73 = 100): 21% more face-up area for the same weight
Face-up size is the outline's area per carat, measured from each model (T73 = 100): 121 for the Rectangle 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.
Rectangle Cut · 8.44 x 5.60 mm at 1.00 ct · Face-up size 121
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
Brilliance 24 against 72 for the T73 Round Brilliant in iolite: 48 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 Rectangle Cut returns 24 of 100 and the T73 72. The Rectangle Cut's pavilion mains sit at 33.5°, the T73's at 40.8°. The maps below show where each stone's light returns from and where it leaks.
Rectangle Cut in iolite · Brilliance 24
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 72 against 28 for the T73 Round Brilliant in iolite: 44 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 Rectangle Cut leaks 72% in iolite against 28% for the T73 in iolite. Grey in the maps below is leaking light; the share under each picture is that picture's own.
Rectangle Cut in iolite · Leakage 72
T73 Round Brilliant in iolite · Leakage 28
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
Fire 60 against 77 for the Rectangle Cut in diamond: 17 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 Rectangle Cut reads 60 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 Rectangle Cut 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.
Scintillation 63 against 80 for the T73 Round Brilliant in iolite: 17 points lower
Scintillation follows the number of facets the stone shows: 100 × (1 − e^(−facets / 45)). The Rectangle Cut has 45 facets and the T73 73, so 63 against 80. It depends on the geometry alone, so it is the same in every stone.
Rectangle Cut · 45 facets · Scintillation 63
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
Pick the Rectangle Cut in iolite for this: it weighs less than in diamond and faces up larger for its weight. The catch: it returns less light than the T73, leaks more light than the T73, shows less fire than in diamond and has fewer facets to sparkle with.
If that catch matters more to you, the DG-M5 reads 85 for brilliance 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 Rectangle Cut model we traced.
In our census the Rectangle Cut returns 24 of 100 for brilliance and 60 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 Rectangle Cut's pavilion mains sit at 33.5°.
At 6.5 mm across, 1.56 ct in iolite, against 2.10 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 Rectangle Cut as exact geometry: 45 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The Rectangle Cut as a faceting design: 41 + 4 girdles = 45 facets, 96 index, 2-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 Rectangle Cut: specifications, model and files