Brilliance 45 · Fire 43 · Scintillation 78 · in our census, where the T73 scores 72 · 58 · 80 in iolite
How the Princess Cut handles light in iolite, measured: brilliance 45, fire 43 and 51% 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 Princess Cut in iolite live.
The Princess Cut has 85 facets and a total depth of 68.5% of its width; its pavilion mains sit at 41.9°. In iolite, the census returns 45 of 100 for brilliance and 43 for fire, against 72 and 58 for the T73, our benchmark, in the same stone; the same Princess Cut in diamond reads 57 and 83.
Compared with the T73 Round Brilliant in iolite and with the Princess 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 Princess 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 1, level on 1, behind on 6.
1.10 ct at 6.5 mm, against 1.48 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 1.10 ct here and 1.48 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the Princess Cut page
Fire 43 against 83 for the Princess Cut in diamond: 40 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 Princess Cut reads 43 against 83 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 Princess 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.
Leakage 51 against 18 for the Princess Cut in diamond: 33 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 Princess Cut leaks 51% in iolite against 18% for the same cut in diamond. Grey in the maps below is leaking light; the share under each picture is that picture's own.
Princess Cut in iolite · Leakage 51
Princess Cut in diamond · Leakage 18
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 45 against 72 for the T73 Round Brilliant in iolite: 27 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 Princess Cut returns 45 of 100 and the T73 72. The Princess Cut's pavilion mains sit at 41.9°, the T73's at 40.8°. The maps below show where each stone's light returns from and where it leaks.
Princess Cut in iolite · Brilliance 45
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
Tilt 82 against 89 for the Princess Cut in diamond: 7 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 Princess Cut keeps 82 against 89 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.
Princess Cut in iolite · Tilt 82
Princess Cut in diamond · Tilt 89
What helps: the T105 Round Brilliant reads 87 for tilt in iolite, the best of the cuts we score there.
Try it yourself: Compare the Tilt spoke in the Cut Performance Wheel
Pick the Princess Cut in iolite for this: it weighs less than in diamond. The catch: it shows less fire than in diamond, leaks more than in diamond, returns less light than the T73 and loses more light when tilted than in diamond.
If that catch matters more to you, the Oval Cut reads 82 for fire 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 Princess Cut model we traced.
In our census the Princess Cut returns 45 of 100 for brilliance and 43 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 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 Princess Cut's pavilion mains sit at 41.9°.
At 6.5 mm across, 1.10 ct in iolite, against 1.48 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 Princess Cut as exact geometry: 93 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The Princess Cut as a faceting design: 81 + 4 girdles = 85 facets, 96 index, 4-fold mirror-image symmetry, ready for the machine.
Save with a bundle: the Princess Cut 3D files are also in Jewelry Essentials 3D, 10 cuts for €29.99 until 1 December, then €39.99 (€99.90 as single files). Buy the bundle.
Save with a bundle: the Princess Cut cutting files are also in Faceter's Classics, 10 cuts for €22.49 until 1 December, then €29.99 (€69.90 as single files). Buy the bundle.
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 Princess Cut: specifications, model and files