Cutting amethyst (quartz)
Amethyst is purple quartz. Quartz has one of the lowest refractive indices of the common faceting materials. Its critical angle is large, so designs made for diamond window badly unless the cutter makes the pavilion steeper. It is also a favorite first stone for cutters, and designers test most faceting designs in quartz first.
Amethyst often shows color zoning. Cutters orient the rough so the darker zone sits low in the pavilion, where the facets spread its color across the face-up stone.
Amethyst 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 amethyst
In amethyst, the DG-M5 Cut performs best of the 11 brilliants we score: 85 for brilliance, 50 for fire and 85 for scintillation. The DG-Anteros Cut follows at 82, 54 and 80, then the DG-Vitruvio Cut at 71, 36 and 98. Of the three, the DG-M5 Cut has the highest brilliance and the DG-Anteros Cut has the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| DG-M5, House cut | 85 (T73 benchmark 72) | 50 (T73 benchmark 36) | 85 (T73 benchmark 80) |
| DG-Anteros, Brilliant | 82 (T73 benchmark 72) | 54 (T73 benchmark 36) | 80 (T73 benchmark 80) |
| DG-Vitruvio, Brilliant | 71 (T73 benchmark 72) | 36 (T73 benchmark 36) | 98 (T73 benchmark 80) |
| DG-Broken Hearts and Arrows, Brilliant | 71 (T73 benchmark 72) | 35 (T73 benchmark 36) | 86 (T73 benchmark 80) |
| T73 Round Brilliant, Benchmark | 72 | 36 | 80 |
| T105 Round Brilliant, Brilliant | 65 (T73 benchmark 72) | 31 (T73 benchmark 36) | 90 (T73 benchmark 80) |
| DG-Miliano, House cut | 60 (T73 benchmark 72) | 28 (T73 benchmark 36) | 98 (T73 benchmark 80) |
| T57 Round Brilliant, Historic cut | 71 (T73 benchmark 72) | 37 (T73 benchmark 36) | 72 (T73 benchmark 80) |
| Old European, Historic cut | 59 (T73 benchmark 72) | 37 (T73 benchmark 36) | 81 (T73 benchmark 80) |
| DG-Rafaello, House cut | 45 (T73 benchmark 72) | 26 (T73 benchmark 36) | 96 (T73 benchmark 80) |
In amethyst, the Oval Cut performs best of the 23 jewelry cuts we score: 52 for brilliance, 69 for fire and 87 for scintillation. The Teardrop Cut follows at 49, 60 and 81, then the Pear Cut at 52, 60 and 76.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| Oval, Jewelry cut | 52 (T73 benchmark 72) | 69 (T73 benchmark 36) | 87 (T73 benchmark 80) |
| Teardrop, Jewelry cut | 49 (T73 benchmark 72) | 60 (T73 benchmark 36) | 81 (T73 benchmark 80) |
| Pear, Jewelry cut | 52 (T73 benchmark 72) | 60 (T73 benchmark 36) | 76 (T73 benchmark 80) |
| Asscher, Jewelry cut | 60 (T73 benchmark 72) | 52 (T73 benchmark 36) | 72 (T73 benchmark 80) |
| Trillion, Jewelry cut | 60 (T73 benchmark 72) | 44 (T73 benchmark 36) | 80 (T73 benchmark 80) |
| Cushion, Jewelry cut | 69 (T73 benchmark 72) | 24 (T73 benchmark 36) | 85 (T73 benchmark 80) |
| Square, Jewelry cut | 65 (T73 benchmark 72) | 34 (T73 benchmark 36) | 78 (T73 benchmark 80) |
| Trilliant, Jewelry cut | 63 (T73 benchmark 72) | 34 (T73 benchmark 36) | 79 (T73 benchmark 80) |
| Emerald Square, Jewelry cut | 61 (T73 benchmark 72) | 40 (T73 benchmark 36) | 72 (T73 benchmark 80) |
| Marquise, Jewelry cut | 38 (T73 benchmark 72) | 48 (T73 benchmark 36) | 86 (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 amethyst. 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.
Amethyst: questions
What pavilion angle should I use for amethyst?
For a round design, at least 40.4°. At that angle or steeper, light that arrives straight down reflects off both sides of the pavilion, beyond the critical angle of amethyst (40.4°). 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 amethyst?
40.4°, from a refractive index of 1.544. 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 amethyst?
We add brilliance, fire and scintillation together. By that total, the DG-M5 Cut, the DG-Anteros Cut and the DG-Vitruvio Cut are the best brilliants in amethyst. By the same total, the best jewelry cuts are the Oval Cut, the Teardrop Cut and the Pear Cut. We score 37 cuts in all, and this page ranks the top ten of each.
How much does amethyst weigh compared with diamond?
Amethyst has a specific gravity of about 2.65, and diamond has 3.52. So a stone of the same size weighs about 0.75 times as much as the diamond.