Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate. It is very widely mined and used in a huge variety of industries from Ancient Egypt to modern day. Gypsum is used in everything from drywall to fertilizer, in removing pollutants from water, and even in baking.
Gypsum can also crystallize and be found as translucent crystals of selenite. The fine-grained white variety of gypsum called alabaster is prized for its ornamental uses. It is soft and often used for carving.
Another form of gypsum is the desert rose, a flower-like form, typically opaque, with embedded sand grains.
Selenite is a crystal habit variety of the mineral gypsum. All varieties of gypsum are composed of calcium sulfate dihydrate.
Selenite is known for creating some of the largest crystals ever found. It is most often transparent and colorless, but can vary with the presence of other minerals.