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. Selenite contains no significant selenium; the similarity of names comes from both substances being named from the Ancient Greek word for the moon. Selenite is rated at a 2 on the Mohs scale for hardness. Selenite is thought to calm irritation and hyperactivity, as well as protect against loss of control.