Cutting diamond
Diamond has the highest refractive index of any natural gem in common use. So its critical angle is small, and almost any sensible pavilion reflects head-on light. A good diamond cut differs from an average one in how much of that light comes back toward the viewer. It also differs in how it breaks that light into brilliance and fire.
Diamond is not cut on a hobby faceting machine. Cutters saw it, brute it and polish it on a scaife, or cut it with lasers. Faceting files are still the standard way to plan and communicate a diamond design, and jewelry CAD needs 3D models.
Diamond optical properties
Where a property has a range, we calculate angles from the lower refractive index. It gives the stricter limit.
The top ten cuts in diamond
In diamond, the DG-Miliano Cut performs best of the 11 brilliants we score: 95 for brilliance, 82 for fire and 98 for scintillation. The DG-Rafaello Cut (94, 80 and 96) and the DG-Vitruvio Cut (93, 79 and 98) share second place. Of the three, the DG-Miliano Cut has both the highest brilliance and the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| DG-Miliano, House cut | 95 (T73 benchmark 93) | 82 (T73 benchmark 78) | 98 (T73 benchmark 80) |
| DG-Rafaello, House cut | 94 (T73 benchmark 93) | 80 (T73 benchmark 78) | 96 (T73 benchmark 80) |
| DG-Vitruvio, Brilliant | 93 (T73 benchmark 93) | 79 (T73 benchmark 78) | 98 (T73 benchmark 80) |
| T105 Round Brilliant, Brilliant | 95 (T73 benchmark 93) | 77 (T73 benchmark 78) | 90 (T73 benchmark 80) |
| DG-Broken Hearts and Arrows, Brilliant | 93 (T73 benchmark 93) | 78 (T73 benchmark 78) | 86 (T73 benchmark 80) |
| T73 Round Brilliant, Benchmark | 93 | 78 | 80 |
| DG-M5, House cut | 97 (T73 benchmark 93) | 67 (T73 benchmark 78) | 85 (T73 benchmark 80) |
| DG-Anteros, Brilliant | 95 (T73 benchmark 93) | 74 (T73 benchmark 78) | 80 (T73 benchmark 80) |
| Old European, Historic cut | 90 (T73 benchmark 93) | 74 (T73 benchmark 78) | 81 (T73 benchmark 80) |
| T57 Round Brilliant, Historic cut | 92 (T73 benchmark 93) | 79 (T73 benchmark 78) | 72 (T73 benchmark 80) |
In diamond, the Portuguese Bird's Nest Cut performs best of the 23 jewelry cuts we score: 85 for brilliance, 65 for fire and 98 for scintillation. The Trilliant Cut follows at 78, 83 and 79, then the Marquise Cut at 64, 89 and 86. Of the three, the Portuguese Bird's Nest Cut has the highest brilliance and the Marquise Cut has the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| Portuguese Bird's Nest, Jewelry cut | 85 (T73 benchmark 93) | 65 (T73 benchmark 78) | 98 (T73 benchmark 80) |
| Trilliant, Jewelry cut | 78 (T73 benchmark 93) | 83 (T73 benchmark 78) | 79 (T73 benchmark 80) |
| Marquise, Jewelry cut | 64 (T73 benchmark 93) | 89 (T73 benchmark 78) | 86 (T73 benchmark 80) |
| Cushion, Jewelry cut | 80 (T73 benchmark 93) | 73 (T73 benchmark 78) | 85 (T73 benchmark 80) |
| Square, Jewelry cut | 85 (T73 benchmark 93) | 72 (T73 benchmark 78) | 78 (T73 benchmark 80) |
| Trillion, Jewelry cut | 67 (T73 benchmark 93) | 87 (T73 benchmark 78) | 80 (T73 benchmark 80) |
| DG-Octagonal Kite, House cut | 88 (T73 benchmark 93) | 82 (T73 benchmark 78) | 60 (T73 benchmark 80) |
| Oval, Jewelry cut | 47 (T73 benchmark 93) | 92 (T73 benchmark 78) | 87 (T73 benchmark 80) |
| Asscher, Jewelry cut | 69 (T73 benchmark 93) | 80 (T73 benchmark 78) | 72 (T73 benchmark 80) |
| Emerald Square, Jewelry cut | 64 (T73 benchmark 93) | 84 (T73 benchmark 78) | 72 (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, with the same figures as the Cut Performance Wheel, where you can compare any cut with any other in diamond. Mazarin, Peruzzi and Old Mine, historic cushions, are in neither list. Each measured cut’s own page shows its figures in all seven gemstones. A simulation of the geometry, not a grading report.
Diamond: questions
What pavilion angle should I use for diamond?
For a round design, at least 38.9°. At that angle or steeper, light that arrives straight down reflects off both sides of the pavilion, beyond the critical angle of diamond (24.4°). 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 diamond?
24.4°, from a refractive index of 2.417. 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 diamond?
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 diamond. By the same total, the best jewelry cuts are the Portuguese Bird's Nest Cut, the Trilliant Cut and the Marquise Cut. We score 37 cuts in all, and this page ranks the top ten of each.