Brilliance 65 · Fire 60 · Scintillation 78 · in our census, where the T73 scores 72 · 58 · 80 in iolite
How the Square Cut handles light in iolite, measured: brilliance 65, fire 60 and 29% 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 Square Cut in iolite live.
The Square Cut has 69 facets and a total depth of 65.0% of its width; its pavilion mains sit at 41.4°. In iolite, the census returns 65 of 100 for brilliance and 60 for fire, against 72 and 58 for the T73, our benchmark, in the same stone; the same Square Cut in diamond reads 85 and 72.
Compared with the T73 Round Brilliant in iolite and with the Square 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 Square 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.
0.85 ct at 6.5 mm, against 1.14 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 0.85 ct here and 1.14 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the Square Cut page
Brilliance 65 against 85 for the Square Cut in diamond: 20 points lower
The same cut in diamond returns 85. 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 Square Cut's pavilion returns less of iolite's light than of diamond's, as the two maps show.
Square Cut in iolite · Brilliance 65
Square Cut in diamond · Brilliance 85
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 29 against 10 for the Square Cut in diamond: 19 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 Square Cut leaks 29% in iolite against 10% for the same cut in diamond. Grey in the maps below is leaking light; the share under each picture is that picture's own.
Square Cut in iolite · Leakage 29
Square Cut in diamond · Leakage 10
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 72 for the Square Cut in diamond: 12 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 Square Cut reads 60 against 72 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 Square 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.
Pick the Square Cut in iolite for this: it weighs less than in diamond. The catch: it returns less light than in diamond, leaks more than in diamond and shows less fire than in diamond.
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 Square Cut model we traced.
In our census the Square Cut returns 65 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 1 pro 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 Square Cut's pavilion mains sit at 41.4°.
At 6.5 mm across, 0.85 ct in iolite, against 1.14 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 Square Cut as exact geometry: 69 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The Square Cut as a faceting design: 57 + 12 girdles = 69 facets, 64 index, 4-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 Square Cut: specifications, model and files