Andradite
Andradite is a species of garnet named after the Brazilian Mineralogist J.B. D'Andrada e Silva (1763-1838).
Varieties of andradite include:
Demantoid: The most valuable garnet with a sub-adamantine lustre (demantoid means diamond-like lustre) and a dispersion value exceeding that of diamond. The colour ranges from light yellowish-green to deep green and a similar grass green to emerald (when traces of chromium are present). The high level of dispersion is somewhat masked by the green colour of the stone.
Demantoid was originally discovered during the mid-to-late 1800's, in the gem-rich Ural Mountains of Russia. Mining ceased following the Bolshevik revolution in 1917 and only recommenced at the beginning of the new millennium. New deposits have been found and once again the Russian source location is back on the map. Small amounts of high quality stones have appeared on the gem market and hopefully will continue.
Stones from some localities (notably Russia) may have radiating fibrous inclusions called 'horsetails' and when present are an identifying feature. Horsetail inclusions are desirable and stones are cut to display them, unlike most inclusions where stones are cut to reduce their visibility.
The highest quality Russian stones are emerald green in colour and command very high prices. Demantoid from locations other than Russia tend to be a yellowish-green colour, but still command high prices.
Deposits of demantoid garnet are found in: China, Korea, Mexico, Russia and USA.
Melanite: Melanite is Greek for black. It is an opaque variety of andradite, used as mourning jewellery.
Deposits of melanite garnet are found in: Europe (France, Germany and Italy) and USA (Colorado).
Topazolite: A yellow to lemon coloured variety resembling topaz, but crystals are small and not commonly worked.
Deposits of topazolite garnet are found in: Italy, Switzerland and USA (California).
Properties of the Gemstone and Additional Information
| Gemstone type: | A species of the garnet group |
| Crystal System: | Cubic (isometric) |
| Chemical Composition: | Calcium iron silicate, Ca3Fe2(SiO4)3 |
| Colour: | As described above for each variety. |
| Lustre: | Bright vitreous: Sub-adamantine for the variety called demantoid |
| Pleochroism: | Not present |
| Dispersion: | High for demantoid (higher than diamond) |
| Hardness: | 6 1/2 on the Mohs' scale |
| Toughness: | Good |
| Cleavage: | Indistinct |
| Density in gm/cc: | 3.82 to 3.85 |
| Double refraction: | None |
| Refractive index: | 1.89 |
Common Treatments
Andradite varieties are not normally treated