Selenite

Gypsum is the most common sulfate mineral. It forms especially in marine evaporite deposits, and also in dry caves, at fumaroles and occasionally in the oxidized zones of sulfide deposits. The best-known selenite occurrence is the Cave of the Crystals at Naica, Chihuahua, Mexico, where the largest crystal found measures about 39 × 13 feet (about 12 × 4 m) and weighs more than 13 tons; the cave was re-flooded and closed in 2015. Other gem-quality gypsum sources include many US states, Australia, Canada, Chile, China, the Czech Republic, Egypt, Germany, Peru, Russia and Spain.

Gypsum varieties differ in habit, not chemistry: selenite is transparent and crystalline, satin spar is fibrous with a silky lustre (some satin spar cabochons show a pronounced cat's-eye), and alabaster is fine-grained and massive. The name satin spar is also used for fibrous calcite and aragonite, so it is best paired with a mineral name. Gypsum is separated from calcite by its lower specific gravity (2.30–2.33 against 2.71–2.94) and lower hardness (2 against 3). Selenite crystals often show zoned "hourglass" fluorescence. Gypsum takes dyes and coatings easily and has been used in assembled cabochons imitating rainbow moonstone.

Gypsum is not recommended for jewellery use: it is very easily scratched (Mohs 2), has perfect and easy cleavage, dissolves slightly in water and is very sensitive to heat, turning white as it loses water. Never use mechanical cleaners that rely on heat or shaking (such as ultrasonic cleaners). Clean it by hand with warm water only, without detergents, and pat it dry rather than rubbing or scrubbing.

The name comes from the Greek selēnē, "moon". Mindat relates it to the moon-like white reflections of the mineral, or to the quality of light passing through translucent gypsum cleavage slabs that were used as windows; the IGS links it to the pearly lustre on cleavage surfaces. J. G. Wallerius introduced the name as "selenites" in his 1747 book Mineralogia, eller Mineralriket.

The Japanese retailer Pascle writes that selenite is said to sharpen its wearer's intuition and decisiveness, is believed to have strong purifying and soothing qualities, and recommends it as a charm for relaxation.