These samples of selenite are displayed in the Smithsonian Museum of Natural History. Selenite is a name given to clear crystals of gypsum, as contrasted with the milky, translucent gypsum that usually forms as evaporative deposits. The name comes from the Greek selene, meaning "moon", a reference to the whitish reflections off the crystal faces. Selenite exhibits beautiful crystals, but they are too soft to be considered as gemstones.
The columns above left are 40 to 50 cm long. That sample is from Maravilla mine, Naica, Chihuahua, Mexico. The sample above right is about 10x16 cm and is from Agrigento, Sicilia, Italy. It is formed with sulfur. The composition of gypsum is CaSO4.2H2O.