Selenite is a crystallized variation of gypsum. It is a hydrous calcium sulfate that is found in many places on earth. Mexico is considered one of the largest exporters of selenite. It is used in fertilizer, drywall, and sidewalk chalk. Some of the largest crystals ever found were composed of selenite, reaching almost 40 feet and weighing over 110,000 pounds. Other variations include desert roses and gypsum flowers.