Other forms include satin spar which has a fibrous appearance, however it is not dangerous like asbestos. The crystals often contain impurities in the form of an hourglass shape in the crystal. The crystals from Nanton, Alberta are phosphorescent in an hourglass shape. The name gypsum originated from the Greek and means calcined or burned material. Selenite also comes from the Greek and relates the pearly luster on the crystals to moonlight.
Gypsum occurs in sedimentary rocks primarily.
It forms in evaporite sequences as a result of land locked oceans drying up. Occasionally gypsum is also observed in low temperature hydrothermal veins. Gypsum is a common mineral and is found in numerous localities. Famous localities include hourglass selenite from Jet, Oklahoma, selenite from the Cave of Swords mine in Naica, Chihuahua, Mexico, Red River Floodway, Winnipeg, Manitoba, and Nanton, Alberta.