Natural Selenite is a transparent crystalline variety of Gypsum composed of hydrated calcium sulfate, CaSO₄·2H₂O. With Mohs hardness 2, perfect cleavage and a vitreous-to-pearly lustre, Selenite is primarily valued for mineral specimens, decorative crystals and spiritual collections. White fibrous material commonly marketed as Selenite is more accurately classified as Satin Spar Gypsum.