Cutting GGG
GGG has a refractive index of 1.970 and a dispersion of 0.045, a specific gravity of 7.05 and a hardness of 6.5 (6 1/2) on the Mohs scale (cubic crystal system). It is singly refractive. Its critical angle, from the low end of the range, is 30.5°, so a round design needs a pavilion of at least 37.4° for head-on light to reflect off both sides of the pavilion and leave through the table.
We ray trace every cut in GGG with the same light census that scores the other gemstones: one refractive index, 1.97, the midpoint of the published range, and a fire spread of 0.039, 0.86 of the published dispersion, the ratio our seven reference stones follow. Nothing is tuned to a result. Our instrument tools model GGG too, except the dichroscope, polariscope and spectroscope, which need a measured spectrum; its figures live on this page, on each cut's page in GGG, in the Cut Performance Wheel and in the Cut Performance Index.
GGG optical properties
Published values, as on the GGG 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 ggg
In ggg, the Cushion Cut performs best of the 23 jewelry cuts we score: 91 for brilliance, 80 for fire and 85 for scintillation. The Trilliant Cut follows at 86, 89 and 79, then the Oval Cut at 66, 96 and 87. Of the three, the Cushion Cut has the highest brilliance and the Oval Cut has the most fire.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| Cushion, Jewelry cut | 91 (T73 benchmark 98) | 80 (T73 benchmark 76) | 85 (T73 benchmark 80) |
| Trilliant, Jewelry cut | 86 (T73 benchmark 98) | 89 (T73 benchmark 76) | 79 (T73 benchmark 80) |
| Oval, Jewelry cut | 66 (T73 benchmark 98) | 96 (T73 benchmark 76) | 87 (T73 benchmark 80) |
| Portuguese Bird's Nest, Jewelry cut | 80 (T73 benchmark 98) | 67 (T73 benchmark 76) | 98 (T73 benchmark 80) |
| Trillion, Jewelry cut | 73 (T73 benchmark 98) | 90 (T73 benchmark 76) | 80 (T73 benchmark 80) |
| Square, Jewelry cut | 81 (T73 benchmark 98) | 77 (T73 benchmark 76) | 78 (T73 benchmark 80) |
| Marquise, Jewelry cut | 60 (T73 benchmark 98) | 89 (T73 benchmark 76) | 86 (T73 benchmark 80) |
| DG-Octagonal Kite, House cut | 88 (T73 benchmark 98) | 84 (T73 benchmark 76) | 60 (T73 benchmark 80) |
| Princess, Jewelry cut | 66 (T73 benchmark 98) | 86 (T73 benchmark 76) | 78 (T73 benchmark 80) |
| Teardrop, Jewelry cut | 59 (T73 benchmark 98) | 89 (T73 benchmark 76) | 81 (T73 benchmark 80) |
In ggg, the DG-Rafaello Cut and the DG-Vitruvio Cut share first place of the 11 brilliants we score. Their scores for brilliance, fire and scintillation are 98, 78 and 96 and 98, 76 and 98. The DG-Miliano Cut follows at 96, 78 and 98.
| Cut | Brilliance | Fire | Scintillation |
|---|---|---|---|
| DG-Rafaello, House cut | 98 (T73 benchmark 98) | 78 (T73 benchmark 76) | 96 (T73 benchmark 80) |
| DG-Vitruvio, Brilliant | 98 (T73 benchmark 98) | 76 (T73 benchmark 76) | 98 (T73 benchmark 80) |
| DG-Miliano, House cut | 96 (T73 benchmark 98) | 78 (T73 benchmark 76) | 98 (T73 benchmark 80) |
| T105 Round Brilliant, Brilliant | 97 (T73 benchmark 98) | 75 (T73 benchmark 76) | 90 (T73 benchmark 80) |
| DG-Broken Hearts and Arrows, Brilliant | 98 (T73 benchmark 98) | 77 (T73 benchmark 76) | 86 (T73 benchmark 80) |
| T73 Round Brilliant, Benchmark | 98 | 76 | 80 |
| T57 Round Brilliant, Historic cut | 98 (T73 benchmark 98) | 77 (T73 benchmark 76) | 72 (T73 benchmark 80) |
| DG-Anteros, Brilliant | 89 (T73 benchmark 98) | 74 (T73 benchmark 76) | 80 (T73 benchmark 80) |
| Old European, Historic cut | 87 (T73 benchmark 98) | 69 (T73 benchmark 76) | 81 (T73 benchmark 80) |
| DG-M5, House cut | 88 (T73 benchmark 98) | 63 (T73 benchmark 76) | 85 (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.
GGG: questions
What pavilion angle should I use for GGG?
For a round design, at least 37.4°. At that angle or steeper, light that arrives straight down reflects off both sides of the pavilion, beyond the critical angle of GGG (30.5°). 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 GGG?
30.5°, from a refractive index of 1.970, 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 GGG?
We add brilliance, fire and scintillation together. By that total, the DG-Rafaello Cut, the DG-Vitruvio Cut and the DG-Miliano Cut are the best brilliants in ggg. By the same total, the best jewelry cuts are the Cushion Cut, the Trilliant Cut and the Oval Cut. We score 37 cuts in all, and this page ranks the top ten of each.
How much does GGG weigh compared with diamond?
GGG has a specific gravity of about 7.05, and diamond has 3.52. So a stone of the same size weighs about 2.00 times as much as the diamond.