Selenite is a transparent crystalline form of gypsum, hydrated calcium sulphate, and it is very soft, hardness 2, scratched by a fingernail, with one perfect cleavage that lets it be split into thin flexible sheets; selenite windows of cleaved gypsum were used as glazing before glass was cheap. The name is from Greek selene, moon, given for the soft moon-like sheen of the satin-spar variety, whose fibrous structure produces a travelling band of light. A point worth stating plainly against much modern crystal literature: selenite is water-soluble and will slowly dissolve or lose its polish if washed, so the common advice to cleanse it in water is mistaken for the material. The great gypsum crystals of the Naica cave in Mexico are selenite. Its twentieth-century metaphysical reputation for clarity and cleansing is recent and name-led; the mineral carries no therapeutic property.
The largest selenite crystals known were found only in the year 2000, in the Cueva de los Cristales, the Cave of Crystals, some three hundred metres below the Naica mine in Chihuahua, Mexico, where beams of gypsum up to twelve metres long grew over half a million years in water held at a near-constant fifty-eight degrees Celsius; the geologist Juan Manuel Garcia-Ruiz led the scientific study of the cave after miners broke into it. The cave's extreme heat and humidity make it lethal to enter without protective cooling suits, and it has since been allowed to flood again, sealing the crystals from further study or disturbance.