Genthelvite
Genthelvite is a mineral that is an extremely unusual gem that forms a mineral series with Helvite and Danalite. Genthelvite is a known member of the Helvine Group of minerals which also includes Danalite, Helvine and Tugtupite. Genthelvite was discovered in Cheyenne Canyon, El Paso County, Colorado, the USA in 1944 and it has since been present in several other locations around the world. Gems are usually very small, less than a quarter carat, and included. Colors include colorless, yellow, green, pink to red and may darken to black and brown from weathering.
The most locations that are notable gem quality crystals are Poudrette quarry Mont Saint-Hilaire, Rouville County, Québec, Canada; Bakstevalåsen, Gunhildrud, Øvre Eiker, Buskerud, Norway; Zagi Mountain, Mulla Ghori, Khyber Agency, Federally Administered Tribal Areas (FATA), Pakistan; West Cheyenne Canyon, near St. Peters Dome, El Paso County, Colorado, United States Of America. These areas produce only really crystals that are tiny.
| Chemical Formula: | Zn4Be3(SiO4)3S |
| Crystallography: | Isometric – Hextetrahedral |
| Crystal Habit: | Crystals commonly tetrahedral, up to 5 cm in diameter; also as large rounded grains |
| Twinning: | None |
| Cleavage: | [111] Poor |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Hardness (Mohs): | 6.0 – 6.5 |
| Density: | 3.44 – 3.70 (g/cm3) |
| Luminescence: | Strong green fluorescence in LW and SW UV; short-lived phosphorescence |
| Radioactivity: | Not Radioactive |
| Other: | Gelatinizes with acids |
| Color: | Red, Pink, Colourless, White, Yellow, Pale Green to Emerald Green; darkens to Brown and Black on weathering |
| Transparency: | Transparent, Translucent |
| Luster: | Vitreous |
| Refractive Index: | 1.738 – 1.752 Isotropic |
| Birefringence: | 0.00 Isotropic |
| Dispersion: | n/a |
| Pleochroism: | None |
