Selenite

Selenite, the translucent crystalline variety of gypsum (q.v.), so named from the resemblance of its pearly lustre to moonlight. It crystallises in the Oblique system, often in flat rhomboid forms, and is in swallow-tailed twins. the crystals are sometimes large, very symmetrical, and laminated parallel to the largest faces, and they may be bent. They are 2 in the scale of hardness. Selenite forms rapidly on the surface of clay from the decomposition of iron pyrites and calcareous fossils in moist air.