Cutting ruby (corundum)
Ruby is red corundum. Its refractive index is far lower than diamond's, so its critical angle is much larger. A pavilion cut for diamond can let light straight out of the back. Corundum designs usually have a steeper pavilion.
Ruby is pleochroic: it shows a purer red along the optic axis and a more orange red across it. Cutters orient the table to the axis when the rough allows it. The shape of the rough often decides between an oval, a cushion or a round.
Ruby 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 ruby
In ruby, the DG-Vitruvio Cut performs best of the 11 brilliants we score: 94 for brilliance, 37 for fire and 98 for scintillation. The DG-M5 Cut follows at 100, 39 and 85, then the DG-Miliano Cut at 90, 35 and 98. Of the three, the DG-M5 Cut has both the highest brilliance and the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| DG-Vitruvio, Brilliant | 94 (T73 benchmark 94) | 37 (T73 benchmark 37) | 98 (T73 benchmark 80) |
| DG-M5, House cut | 100 (T73 benchmark 94) | 39 (T73 benchmark 37) | 85 (T73 benchmark 80) |
| DG-Miliano, House cut | 90 (T73 benchmark 94) | 35 (T73 benchmark 37) | 98 (T73 benchmark 80) |
| DG-Anteros, Brilliant | 97 (T73 benchmark 94) | 43 (T73 benchmark 37) | 80 (T73 benchmark 80) |
| DG-Broken Hearts and Arrows, Brilliant | 95 (T73 benchmark 94) | 36 (T73 benchmark 37) | 86 (T73 benchmark 80) |
| T105 Round Brilliant, Brilliant | 90 (T73 benchmark 94) | 36 (T73 benchmark 37) | 90 (T73 benchmark 80) |
| T73 Round Brilliant, Benchmark | 94 | 37 | 80 |
| DG-Rafaello, House cut | 85 (T73 benchmark 94) | 25 (T73 benchmark 37) | 96 (T73 benchmark 80) |
| T57 Round Brilliant, Historic cut | 94 (T73 benchmark 94) | 39 (T73 benchmark 37) | 72 (T73 benchmark 80) |
| Old European, Historic cut | 78 (T73 benchmark 94) | 39 (T73 benchmark 37) | 81 (T73 benchmark 80) |
In ruby, the Oval Cut performs best of the 23 jewelry cuts we score: 66 for brilliance, 74 for fire and 87 for scintillation. The Trilliant Cut follows at 86, 52 and 79, then the Trillion Cut at 72, 59 and 80. Of the three, the Trilliant Cut has the highest brilliance and the Oval Cut has the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| Oval, Jewelry cut | 66 (T73 benchmark 94) | 74 (T73 benchmark 37) | 87 (T73 benchmark 80) |
| Trilliant, Jewelry cut | 86 (T73 benchmark 94) | 52 (T73 benchmark 37) | 79 (T73 benchmark 80) |
| Trillion, Jewelry cut | 72 (T73 benchmark 94) | 59 (T73 benchmark 37) | 80 (T73 benchmark 80) |
| Cushion, Jewelry cut | 88 (T73 benchmark 94) | 36 (T73 benchmark 37) | 85 (T73 benchmark 80) |
| Teardrop, Jewelry cut | 59 (T73 benchmark 94) | 64 (T73 benchmark 37) | 81 (T73 benchmark 80) |
| Square, Jewelry cut | 78 (T73 benchmark 94) | 45 (T73 benchmark 37) | 78 (T73 benchmark 80) |
| Princess, Jewelry cut | 68 (T73 benchmark 94) | 52 (T73 benchmark 37) | 78 (T73 benchmark 80) |
| Emerald Square, Jewelry cut | 71 (T73 benchmark 94) | 53 (T73 benchmark 37) | 72 (T73 benchmark 80) |
| Marquise, Jewelry cut | 55 (T73 benchmark 94) | 54 (T73 benchmark 37) | 86 (T73 benchmark 80) |
| Pear, Jewelry cut | 62 (T73 benchmark 94) | 56 (T73 benchmark 37) | 76 (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 ruby. 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.
Ruby: questions
What pavilion angle should I use for ruby?
For a round design, at least 36.4°. At that angle or steeper, light that arrives straight down reflects off both sides of the pavilion, beyond the critical angle of ruby (34.6°). 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 ruby?
34.6°, from a refractive index of 1.762. 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 ruby?
We add brilliance, fire and scintillation together. By that total, the DG-Vitruvio Cut, the DG-M5 Cut and the DG-Miliano Cut are the best brilliants in ruby. By the same total, the best jewelry cuts are the Oval Cut, the Trilliant Cut and the Trillion Cut. We score 37 cuts in all, and this page ranks the top ten of each.
How much does ruby weigh compared with diamond?
Ruby has a specific gravity of about 4.00, and diamond has 3.52. So a stone of the same size weighs about 1.14 times as much as the diamond.