Selenite bends. Cut a thin sheet of it and you can flex it slightly between your fingers without snapping it, a texture almost no other common mineral has, and one that seems to contradict the whole idea of a “crystal” being rigid.
Gypsum’s cleanest form
Selenite is simply a clear, well-formed crystalline variety of gypsum, calcium sulfate dihydrate, distinguished from massive or fibrous gypsum mainly by its transparency and its large, flat crystal faces. Gypsum’s structure is built from sheets of calcium and sulfate ions loosely bonded to layers of water molecules, and that water layer is the reason for both selenite’s slight flexibility and its low hardness, just 2 on the Mohs scale, soft enough to scratch with a fingernail. The same loosely stacked structure gives selenite perfect cleavage in one direction, splitting easily into thin, flexible sheets rather than shattering.
Where it grows large
Selenite crystals can reach enormous sizes under the right conditions, slow evaporation of mineral-rich water in a stable, undisturbed cave environment lets crystals grow for thousands of years without interruption. The most extreme known example, the Cave of Crystals in Naica, Mexico, produced selenite beams over 30 feet long, a scale that took a specific and short-lived combination of geothermal heat and mineral saturation to reach. Our piece on that discovery goes further into how conditions that extreme came together.
Because gypsum dissolves in water far more readily than most minerals, selenite specimens need drier handling and storage than a typical quartz or calcite piece, a practical note covered in our guide to cleaning and storing specimens.
