The gemstone pages and the simulator use these values. For stones with a range, the refractive index used is the lower end, which gives the stricter angle limits. Dispersion is the B to G interval (686.7 to 430.8 nm).
| Gemstone | Refractive index | Birefringence | Dispersion | Specific gravity | Critical angle |
|---|---|---|---|---|---|
| Diamond | 2.417 | None | 0.044 | 3.52 | 24.4° |
| Moissanite | 2.648 to 2.691 | 0.043 | 0.104 | 3.22 | 22.2° |
| Cubic zirconia | 2.15 to 2.18 | None | 0.060 | 5.6 to 6.0 (5.8 used) | 27.7° |
| Ruby | 1.762 to 1.770 | 0.008 | 0.018 | 4.00 | 34.6° |
| Sapphire | 1.762 to 1.770 | 0.008 | 0.018 | 4.00 | 34.6° |
| Emerald | 1.577 to 1.583 | 0.006 | 0.014 | 2.72 | 39.4° |
| Amethyst | 1.544 to 1.553 | 0.009 | 0.013 | 2.65 | 40.4° |
The Brightness & Tilt Analysis and the light census behind the Performance Wheel come from our research engine and use its constants, which differ slightly from the table above:
| Gemstone | Index n | Fire spread Δ | Fresnel F0 |
|---|---|---|---|
| Diamond | 2.417 | 0.038 | 0.172 |
| Moissanite | 2.65 | 0.090 | 0.204 |
| Cubic zirconia | 2.16 | 0.052 | 0.135 |
| Ruby and sapphire | 1.765 | 0.015 | 0.077 |
| Emerald | 1.578 | 0.012 | (census only) |
| Amethyst | 1.548 | 0.011 | (census only) |
The fire spread is about 0.86 of the published dispersion. F0 is the reflectance at normal incidence, ((n−1)/(n+1))².
Critical angle
Light inside the stone meeting a facet at more than the critical angle from its normal reflects totally:
Each gemstone page also gives the shallowest pavilion at which a round returns head-on light. In the meridian section, light arriving straight down meets the first pavilion main at the pavilion angle p, the opposite main at 180° − 3p, and the table at |180° − 4p|. It must reflect at the first and leave at the table:
That is 38.9° for diamond, 36.4° for ruby and sapphire, and the critical angle itself for emerald and amethyst. Faceters usually cut a degree or two steeper, for light that arrives off the axis.
Head-on light on cut pages
“Pavilion reflecting head-on light” is the share of the pavilion, by projected area, whose facets lie at or above the critical angle from the girdle plane. Light arriving straight down meets each such facet at at least θc and reflects. It is a first-reflection test only. The verdict reads “Holds head-on light” at 99% or more, “Minor windowing” from 90%, and “Steepen the pavilion” below that. A culet flat counts as not reflecting, and a cut with no table is not measured.