Demantoid is the green variety of andradite, a garnet species of calcium, iron and silicon. Chromium causes its color.
Properties
| Refractive index | 1.881 to 1.888 (demantoid); andradite 1.855 to 1.95 |
|---|---|
| Birefringence | None (singly refractive) |
| Optic character | Singly refractive |
| Dispersion | 0.057 |
| Specific gravity | 3.82 to 3.88 (demantoid); andradite 3.70 to 4.10 |
| Hardness (Mohs) | 6.5 to 7 |
| Crystal system | Cubic |
| Pleochroism | None |
| Absorption spectrum | Strong band, with cutoff at the violet end, 443 nm; doublet, sometimes visible in demantoid, 701 nm (Cr); sharp line, 693 nm (Cr); band in the orange, 640 nm (Cr); band in the orange, 622 nm (Cr) |
Published values. They change a little from stone to stone. Our tools do not model demantoid yet. Sources: International Gem Society · GIA · Gemdat
In more detail
Andradite is softer than grossular, but it has a higher refractive index and more dispersion. Cutters often use exact proportions to show demantoid's fire. Some demantoids have horsetail inclusions, radiating bundles of mineral fibers, which can raise their value. A horsetail does not prove Russian origin, because demantoids from Iran and Italy can have them too.
Related terms
- Garnet: Garnet is a group of closely related mineral species. They share the same crystal structure but differ in chemical composition.
- Grossular: Grossular is a garnet species made of calcium, aluminum and silicon. It has perhaps the widest color range of any garnet species.
- Dispersion: Dispersion is how differently a stone bends red and violet light. Gemologists give it as the index difference between the B line (686.7 nm) and the G line (430.8 nm).
- Fire: Fire is the flashes of spectral color that a cut stone sends out. In our census, fire is the share of returned light whose colors leave more than 0.6° apart.
- Inclusion: An inclusion is a clarity characteristic inside a diamond, or one that reaches the surface from inside.