透石膏

Selenite is the transparent, well-crystallised variety of gypsum (hydrated calcium sulfate). It forms glassy, colourless to white crystals that can be astonishingly clear, as well as the fibrous, silky form known as satin spar and the bladed rosettes called desert roses.

Identifying selenite

Selenite is very soft — just 2 on the Mohs scale, scratched with a fingernail — and has one perfect cleavage that produces flat, flexible sheets. It is light, often water-clear, with a pearly to glassy lustre. Because it is soft and slightly water-soluble, it needs gentle, dry handling.

Where it is found

Gypsum is widespread in evaporite deposits worldwide. The most spectacular selenite comes from the Naica mine in Chihuahua, Mexico, whose Cave of Crystals holds some of the largest natural crystals ever found — translucent beams metres long. Fine selenite also comes from Morocco, the United States and many other localities.

For collectors

Clarity, crystal size and undamaged terminations drive value; satin spar is widely cut and carved, and desert roses are popular natural ornaments.