Cutting zircon
Zircon has a refractive index of 1.925 to 1.984 and a dispersion of 0.039, a specific gravity of 4.60 to 4.73, usually about 4.70 and a hardness of 7.5 (high zircon); 6 to 7.5 from low to high on the Mohs scale (tetragonal crystal system). It is doubly refractive (uniaxial), with a birefringence of 0.036 to 0.059, usually 0.059. Its critical angle, from the low end of the range, is 31.3°, so a round design needs a pavilion of at least 37.2° for head-on light to reflect off both sides of the pavilion and leave through the table.
We ray trace every cut in zircon with the same light census that scores the other gemstones: one refractive index, 1.955, the midpoint of the published range, and a fire spread of 0.034, 0.86 of the published dispersion, the ratio our seven reference stones follow. Nothing is tuned to a result. Our instrument tools model zircon too, except the dichroscope, polariscope and spectroscope, which need a measured spectrum; its figures live on this page, on each cut's page in zircon, in the Cut Performance Wheel and in the Cut Performance Index.
Zircon is strongly doubly refractive (birefringence up to 0.059), so its back facets can look doubled. Our census traces one index (1.955), so the doubling is not in these figures.
Zircon optical properties
Published values, as on the zircon glossary page; they change a little from stone to stone. Where a property has a range, we calculate angles from the lower refractive index. It gives the stricter limit. Sources: GIA · International Gem Society · Gemdat.
The top ten cuts in zircon
In zircon, the Trilliant Cut performs best of the 23 jewelry cuts we score: 86 for brilliance, 86 for fire and 79 for scintillation. The Cushion Cut follows at 91, 74 and 85, then the Oval Cut at 67, 93 and 87. Of the three, the Cushion Cut has the highest brilliance and the Oval Cut has the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| Trilliant, Jewelry cut | 86 (T73 benchmark 98) | 86 (T73 benchmark 69) | 79 (T73 benchmark 80) |
| Cushion, Jewelry cut | 91 (T73 benchmark 98) | 74 (T73 benchmark 69) | 85 (T73 benchmark 80) |
| Oval, Jewelry cut | 67 (T73 benchmark 98) | 93 (T73 benchmark 69) | 87 (T73 benchmark 80) |
| Portuguese Bird's Nest, Jewelry cut | 80 (T73 benchmark 98) | 62 (T73 benchmark 69) | 98 (T73 benchmark 80) |
| Trillion, Jewelry cut | 73 (T73 benchmark 98) | 87 (T73 benchmark 69) | 80 (T73 benchmark 80) |
| Marquise, Jewelry cut | 61 (T73 benchmark 98) | 87 (T73 benchmark 69) | 86 (T73 benchmark 80) |
| Square, Jewelry cut | 82 (T73 benchmark 98) | 73 (T73 benchmark 69) | 78 (T73 benchmark 80) |
| Princess, Jewelry cut | 67 (T73 benchmark 98) | 83 (T73 benchmark 69) | 78 (T73 benchmark 80) |
| Teardrop, Jewelry cut | 59 (T73 benchmark 98) | 87 (T73 benchmark 69) | 81 (T73 benchmark 80) |
| Asscher, Jewelry cut | 72 (T73 benchmark 98) | 80 (T73 benchmark 69) | 72 (T73 benchmark 80) |
In zircon, the DG-Miliano Cut performs best of the 11 brilliants we score: 96 for brilliance, 73 for fire and 98 for scintillation. The DG-Rafaello Cut (98, 72 and 96) and the DG-Vitruvio Cut (98, 70 and 98) share second place.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| DG-Miliano, House cut | 96 (T73 benchmark 98) | 73 (T73 benchmark 69) | 98 (T73 benchmark 80) |
| DG-Rafaello, House cut | 98 (T73 benchmark 98) | 72 (T73 benchmark 69) | 96 (T73 benchmark 80) |
| DG-Vitruvio, Brilliant | 98 (T73 benchmark 98) | 70 (T73 benchmark 69) | 98 (T73 benchmark 80) |
| T105 Round Brilliant, Brilliant | 96 (T73 benchmark 98) | 68 (T73 benchmark 69) | 90 (T73 benchmark 80) |
| DG-Broken Hearts and Arrows, Brilliant | 98 (T73 benchmark 98) | 69 (T73 benchmark 69) | 86 (T73 benchmark 80) |
| T73 Round Brilliant, Benchmark | 98 | 69 | 80 |
| T57 Round Brilliant, Historic cut | 98 (T73 benchmark 98) | 69 (T73 benchmark 69) | 72 (T73 benchmark 80) |
| DG-Anteros, Brilliant | 89 (T73 benchmark 98) | 70 (T73 benchmark 69) | 80 (T73 benchmark 80) |
| DG-M5, House cut | 88 (T73 benchmark 98) | 61 (T73 benchmark 69) | 85 (T73 benchmark 80) |
| Old European, Historic cut | 86 (T73 benchmark 98) | 65 (T73 benchmark 69) | 81 (T73 benchmark 80) |
Brilliance is the share of face-up light that the stone returns. Fire is the share of that light that splits into color. Both come from our face-up light census. Scintillation is the sparkle that facets give as a stone moves, and we read it from the facets of each cut. We score 37 cuts, the same figures as our Cut Performance Index. The Cut Performance Wheel does not model this stone yet. Mazarin, Peruzzi and Old Mine, historic cushions, are in neither list. Each measured cut’s own page shows its figures in every gemstone we trace. A simulation of the geometry, not a grading report.
Zircon: questions
What pavilion angle should I use for zircon?
For a round design, at least 37.2°. At that angle or steeper, light that arrives straight down reflects off both sides of the pavilion, beyond the critical angle of zircon (31.3°). Then it leaves through the table. The angle is the larger of the critical angle and (180° − critical angle) / 4. Faceters usually cut a degree or two steeper, for light that arrives off the axis. Fancy shapes vary, so every cut page lists its own figure.
What is the critical angle of zircon?
31.3°, from a refractive index of 1.925, the low end of its published range. Light that strikes a facet from inside the stone at less than this angle from the normal of the facet passes out. At a larger angle, it reflects.
Which cuts work best in zircon?
We add brilliance, fire and scintillation together. By that total, the DG-Miliano Cut, the DG-Rafaello Cut and the DG-Vitruvio Cut are the best brilliants in zircon. By the same total, the best jewelry cuts are the Trilliant Cut, the Cushion Cut and the Oval Cut. We score 37 cuts in all, and this page ranks the top ten of each.
How much does zircon weigh compared with diamond?
Zircon has a specific gravity of about 4.67, and diamond has 3.52. So a stone of the same size weighs about 1.33 times as much as the diamond.