Selenite
Selenite is the transparent, glassy variety of gypsum (CaSO4 2H2O), often forming long prismatic or fishtail twin crystals. The Naica giant crystal cave in Mexico is the most famous occurrence.
About Selenite
Selenite is the colorless to white transparent variety of gypsum, a hydrated calcium sulfate. It forms by evaporation in salt lakes, sedimentary basins, and hydrothermal cavities. The most spectacular known occurrence is the Cave of Crystals at Naica, Chihuahua, where megacrystals up to twelve meters long grew over hundreds of thousands of years in a heated brine pool. More accessible commercial sources include Mexico, Morocco, and the United States.
Identification & care
Selenite has a Mohs hardness of only 2 — it can be scratched with a fingernail. Specific gravity is 2.32. It cleaves perfectly in one direction, producing thin sheets that flex slightly without breaking. Other diagnostic features include a pearly luster on cleavage faces, vitreous on broken faces, perfect transparency in clean crystals, and complete water solubility — selenite must never be cleaned with water. Crystals are commonly tabular, prismatic, or twinned in fishtail form.
Collector context
Collector notes
Selenite is among the most affordable transparent collectible minerals because deposits are large and the material is soft enough to fabricate easily. Large terminated single crystals from Morocco and Mexico are common cabinet pieces. Premium material includes Naica giant-crystal-cave specimens (no longer collected for ethical and safety reasons), Spanish fishtail twins, and bladed satin spar from the United Kingdom. Care is critical — never wash with water, avoid humid environments, and handle by the matrix not the cleavage planes.