Selenite:

Selenite is frequently used to indicate well-crystalline Gypsum. Selenite is a crystallized form of gypsum. Chemically, it is a hydrous calcium sulfate. It is a glassy, well-crystallized form of gypsum and is often referred to as satin spar. Water becomes encased in a cavity of some crystals. Many types of selenite crystals are referred to by there shapes: fishtail selenite, hourglass selenite, ruler selenite and desert rose selenite. The type shown here for sale are similar to ruler selenite crystals that appear long and strait with very flat surface. Large caves in mexico have been found, with giant pillar or ruler shaped crystals longer than cars or buses.