Selenite

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.

Selenite is a transparent crystalline variety of gypsum, calcium sulphate dihydrate, and its two most interesting histories are architectural and geological rather than ornamental.

Gypsum crystallises in large, clear, flat plates that cleave into thin sheets, and before glass was cheap those sheets were used as window glazing. The Romans quarried the material, which they called lapis specularis, principally in Hispania, and used it for windows, for lanterns and for the panes of forcing frames in gardens. Pliny describes it, and the mines in what is now the province of Cuenca in Spain have been excavated and are extensive. A gypsum window admits a diffuse light and is not transparent in the way glass is, which is worth knowing when picturing a Roman interior.

The geology produced something more extraordinary. In the Cueva de los Cristales at Naica in Chihuahua, Mexico, a chamber opened by mine dewatering in 2000 was found to contain selenite crystals over ten metres long, among the largest crystals of any mineral ever recorded. They grew slowly in mineral-saturated water held for hundreds of thousands of years at a temperature just below the point at which gypsum converts to anhydrite. The cave is close to unsurvivable for an unprotected person and has since been allowed to reflood.

The name is from Greek selene, the moon, for the pearly sheen on a cleaved surface. Selenite is very soft, two on the Mohs scale, and it is slightly soluble in water, so it cannot be washed.