Selenite (Gypsum)
Gypsum is one of the most abundant minerals but not often faceted. It is very soft and rarely transparent enough for faceting. Selenite is the name given to colorless, transparent Gypsum crystals and faceted gems. Alabaster is the fine grained massive variety that has been used for carving vases, bowls and other decorative items for thousands of years. Satin Spar is the name given to the compact fibrous aggregate type of Gypsum. Plaster of Paris is made by heating Gypsum to about 300 degrees Fahrenheit, removing 75% of the water. Heating Gypsum to about 350 degrees removes the remaining water and converts Gypsum to the mineral Anhydrite.
There are many locations for Gypsum but only a few locations for gem quality Selenite crystals including Rio Grande do Sul, South Region, Brazil; Willow Creek, Nanton, Alberta, Canada; Liupanshui Prefecture, Guizhou Province, China.
| Crystallography: | Monoclinic – Prismatic |
| Crystal Habit: | Crystals as acicular to stubby prismatic crystals, thin to thick tabular, with more than 20 other forms noted, coarsely striated, to 17 m; lenticular in rosettes, may be curved, bent; fibrous, earthy, concretionary, granular, massive. |
| Twinning: | Very common forming cruciform and V-shaped twins; as butterfly or heart-shaped twins. |
| Cleavage: | [010] Perfect, [100] Distinct |
| Fracture: | Splintery parallel to [001], conchoidal on [100] |
| Tenacity: | Flexible, Inelastic |
| Hardness (Mohs): | 1.5 – 2.0 (varies with direction) |
| Density: | 2.317 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioacitve |
| Other: | Slightly soluble in water |
| Color: | Colorless, White; if colored by impurities, Yellow, Tan, Blue, Pink, Brown, Reddish Brown, Gray, Black |
| Transparency: | Transparent to Translucent |
| Luster: | Sub-vitreous, pearly on cleavages, silky if fibrous |
| Refractive Index: | 1.521 – 1.530 Biaxial ( + ) |
| Birefringence: | 0.166 (very high) |
| Dispersion: | Strong; r > v |
| Pleochroism: | n/a |
