A monoclinic mineral, 8(CaSO₄.2H₂O) ; colorless to white in crystals, but massive beds may range from red to yellow to brown, gray, or black; the most common natural sulfate; defines 2 on the Mohs hardness scale; commonly associated with rock salt (halite) and anhydrite; forms beds and lenses interstratified with limestone, shale, and clay, esp. in rocks of Permian to Triassic age; also in volcanic fumarolic deposits; an accessory mineral in metalliferous veins.