Cutting tourmaline
Tourmaline has a refractive index of 1.603 to 1.655 and a dispersion of 0.017, a specific gravity of 3.06 and a hardness of 7 to 7.5 on the Mohs scale (trigonal crystal system). It is doubly refractive (uniaxial), with a birefringence of 0.018 to 0.040. Its critical angle, from the low end of the range, is 38.6°, so a round design needs a pavilion of at least 38.6° for head-on light to reflect off both sides of the pavilion and leave through the table.
We ray trace every cut in tourmaline with the same light census that scores the other gemstones: one refractive index, 1.629, the midpoint of the published range, and a fire spread of 0.015, 0.86 of the published dispersion, the ratio our seven reference stones follow. Nothing is tuned to a result. Our instrument tools model tourmaline too, except the dichroscope, polariscope and spectroscope, which need a measured spectrum; its figures live on this page, on each cut's page in tourmaline, in the Cut Performance Wheel and in the Cut Performance Index.
Tourmaline is strongly doubly refractive (birefringence up to 0.040), so its back facets can look doubled. Our census traces one index (1.629), so the doubling is not in these figures.
Tourmaline optical properties
Published values, as on the tourmaline 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 · LibreTexts Gemology · Gemdat.
The top ten cuts in tourmaline
In tourmaline, the Oval Cut performs best of the 23 jewelry cuts we score: 59 for brilliance, 80 for fire and 87 for scintillation. The Trilliant Cut follows at 77, 53 and 79, then the Trillion Cut at 68, 60 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 | 59 (T73 benchmark 86) | 80 (T73 benchmark 50) | 87 (T73 benchmark 80) |
| Trilliant, Jewelry cut | 77 (T73 benchmark 86) | 53 (T73 benchmark 50) | 79 (T73 benchmark 80) |
| Trillion, Jewelry cut | 68 (T73 benchmark 86) | 60 (T73 benchmark 50) | 80 (T73 benchmark 80) |
| Teardrop, Jewelry cut | 57 (T73 benchmark 86) | 70 (T73 benchmark 50) | 81 (T73 benchmark 80) |
| Cushion, Jewelry cut | 81 (T73 benchmark 86) | 40 (T73 benchmark 50) | 85 (T73 benchmark 80) |
| Square, Jewelry cut | 73 (T73 benchmark 86) | 51 (T73 benchmark 50) | 78 (T73 benchmark 80) |
| Emerald Square, Jewelry cut | 71 (T73 benchmark 86) | 59 (T73 benchmark 50) | 72 (T73 benchmark 80) |
| Pear, Jewelry cut | 59 (T73 benchmark 86) | 67 (T73 benchmark 50) | 76 (T73 benchmark 80) |
| Asscher, Jewelry cut | 65 (T73 benchmark 86) | 58 (T73 benchmark 50) | 72 (T73 benchmark 80) |
| Marquise, Jewelry cut | 49 (T73 benchmark 86) | 60 (T73 benchmark 50) | 86 (T73 benchmark 80) |
In tourmaline, the DG-Vitruvio Cut performs best of the 11 brilliants we score: 85 for brilliance, 51 for fire and 98 for scintillation. The DG-M5 Cut follows at 96, 49 and 85, then the DG-Anteros Cut at 93, 56 and 80. 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-Vitruvio, Brilliant | 85 (T73 benchmark 86) | 51 (T73 benchmark 50) | 98 (T73 benchmark 80) |
| DG-M5, House cut | 96 (T73 benchmark 86) | 49 (T73 benchmark 50) | 85 (T73 benchmark 80) |
| DG-Anteros, Brilliant | 93 (T73 benchmark 86) | 56 (T73 benchmark 50) | 80 (T73 benchmark 80) |
| DG-Broken Hearts and Arrows, Brilliant | 86 (T73 benchmark 86) | 48 (T73 benchmark 50) | 86 (T73 benchmark 80) |
| DG-Miliano, House cut | 76 (T73 benchmark 86) | 45 (T73 benchmark 50) | 98 (T73 benchmark 80) |
| T73 Round Brilliant, Benchmark | 86 | 50 | 80 |
| T105 Round Brilliant, Brilliant | 79 (T73 benchmark 86) | 47 (T73 benchmark 50) | 90 (T73 benchmark 80) |
| T57 Round Brilliant, Historic cut | 85 (T73 benchmark 86) | 50 (T73 benchmark 50) | 72 (T73 benchmark 80) |
| Old European, Historic cut | 71 (T73 benchmark 86) | 49 (T73 benchmark 50) | 81 (T73 benchmark 80) |
| DG-Rafaello, House cut | 66 (T73 benchmark 86) | 35 (T73 benchmark 50) | 96 (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.
Tourmaline: questions
What pavilion angle should I use for tourmaline?
For a round design, at least 38.6°. At that angle or steeper, light that arrives straight down reflects off both sides of the pavilion, beyond the critical angle of tourmaline (38.6°). 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 tourmaline?
38.6°, from a refractive index of 1.603, 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 tourmaline?
We add brilliance, fire and scintillation together. By that total, the DG-Vitruvio Cut, the DG-M5 Cut and the DG-Anteros Cut are the best brilliants in tourmaline. 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 tourmaline weigh compared with diamond?
Tourmaline has a specific gravity of about 3.13, and diamond has 3.52. So a stone of the same size weighs about 0.89 times as much as the diamond.