(Jon of the Clue Crew holds a white rock at White Sands National Monument, New Mexico.)  Because it's water-soluble, this form of calcium sulfate is rarely found in sand, but here in New Mexico's Tularosa Basin, there are no rivers to carry it away, so it forms the famed white sand
... the 
southern portion of the Tularosa Basin from any outlet of water to any sea or ... 
beneath the surface of Lake Lucero, selenite crystals form in the mud, just as they 
... remain the primary source of new dunes in White Sands National Monument.