Brilliance 37 · Fire 33 · Scintillation 81 · in our census, where the T73 scores 72 · 58 · 80 in iolite
How the Old Mine Cut handles light in iolite, measured: brilliance 37, fire 33 and 62% 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 Old Mine Cut in iolite live.
The Old Mine Cut has 74 facets, a table of 60.3% of its width and a total depth of 65.6% of its width; its pavilion mains sit at 44.4°. In iolite, the census returns 37 of 100 for brilliance and 33 for fire, against 72 and 58 for the T73, our benchmark, in the same stone; the same Old Mine Cut in diamond reads 59 and 72.
Compared with the T73 Round Brilliant in iolite and with the Old Mine 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 Old Mine 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, behind on 6.
0.90 ct at 6.5 mm, against 1.21 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 0.90 ct here and 1.21 ct in diamond, 6.5 mm across. Nothing about the light changes with it.
Try it yourself: Weights in every gemstone on the Old Mine Cut page
Fire 33 against 72 for the Old Mine Cut in diamond: 39 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 Old Mine Cut reads 33 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 Old Mine 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.
Brilliance 37 against 72 for the T73 Round Brilliant in iolite: 35 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 Old Mine Cut returns 37 of 100 and the T73 72. The Old Mine Cut's pavilion mains sit at 44.4°, the T73's at 40.8°. The maps below show where each stone's light returns from and where it leaks.
Old Mine Cut in iolite · Brilliance 37
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 62 against 28 for the T73 Round Brilliant in iolite: 34 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 Old Mine Cut leaks 62% 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.
Old Mine Cut in iolite · Leakage 62
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
Symmetry 90 against 100 for the T73 Round Brilliant in iolite: 10 points lower
Symmetry reads how evenly the pavilion facets of each design family are cut: 100 − 6 × their scatter in degrees. The Old Mine Cut reads 90, the T73 100. It depends on the geometry alone, so it is the same in every stone.
Old Mine Cut · 74 facets · Symmetry 90
T73 Round Brilliant · 73 facets · Symmetry 100
Try it yourself: Compare the Symmetry spoke in the Cut Performance Wheel
Pick the Old Mine Cut in iolite for this: it weighs less than in diamond. The catch: it shows less fire than in diamond, returns less light than the T73, leaks more light than the T73 and has a less regular pavilion than the T73.
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 Old Mine Cut model we traced.
In our census the Old Mine Cut returns 37 of 100 for brilliance and 33 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 Old Mine Cut's pavilion mains sit at 44.4°.
At 6.5 mm across, 0.90 ct in iolite, against 1.21 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 Old Mine Cut as exact geometry: 74 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The Old Mine Cut as a faceting design: 58 + 16 girdles = 74 facets, 96 index, 4-fold mirror-image symmetry, ready for the machine.
Save with a bundle: the Old Mine Cut 3D and cutting files are also in Historic Cuts Complete, 6 cuts for €29.99 until 1 December, then €39.99 (€101.88 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 Old Mine Cut: specifications, model and files