Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[3] It is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire and the Nottingham alabasters of Medieval England. Mohs scale of mineral hardness, based on scratch Hardness comparison, defines hardness value 2 as gypsum. It forms as an evaporite mineral and as a hydration product of anhydrite.

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Selenite

Selenite, satin spar, desert rose, and gypsum flower are four varieties of the mineral gypsum; all four varieties show obvious crystalline structure. The four "crystalline" varieties of gypsum are sometimes grouped together and called selenite.

All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no significant selenium; the similarity of names comes from both substances being named from the Ancient Greek word for the Moon.

Some of the largest crystals ever found are selenite crystals - the largest specimen found in the Naica Mine's Cave of the Crystals is 12 metres long and weighs 55 tons.

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Cave of the Crystals

Cave of the Crystals or Giant Crystal Cave (Spanish: Cueva de los Cristales) is a cave connected to the Naica Mine at a depth of 300 metres (980 ft), in Naica, Chihuahua, Mexico.

The main chamber contains giant selenite crystals (gypsum, CaSO4·2 H2O), some of the largest natural crystals ever found. The cave's largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is extremely hot, with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection, people can only endure approximately ten minutes of exposure at a time.

The cave was discovered by the brothers Eloy and Javier Delgado. A group of scientists known as the Naica Project have been heavily involved in researching these caverns.

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