calcium sulfate

Calcium sulfate is an inorganic salt with the chemical formula CaSO4. It occurs in several hydrated forms; the anhydrous state is a white crystalline solid often found in evaporite deposits. Its dihydrate form is the mineral gypsum, which may be dehydrated to produce bassanite, the hemihydrate state. Gypsum occurs in nature as crystals (selenite) or fibrous masses, typically colorless to white, though impurities can impart other hues. All forms of calcium sulfate are sparingly soluble in water and cause permanent hardness when dissolved therein. — Wikipedia