Selenite is hydrous calcium sulphate. Selenite is soft and can easily be scratched with a fingernail. In some very rare instances, water becomes encased in a cavity as the crystal forms. Because of the way Selenite crystals form Selenite exhibits fibre optic qualities. 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.