A designated area of the 11K acres at the Salt Plains National Wildlife Refuge in Oklahoma has gypsum concentrations high enough to grow selenite, a crystalline form of gypsum. The selenite crystals found here have an hourglass-shaped sand inclusion that is not known to occur in selenite crystals found elsewhere in the world. Digging for crystals is only allowed from April - October to protect this vital Whooping Crane habitat. The people are tiny but you can spot their parked cars.