Gypsum

Identification: The mineral gypsum is a hydrous calcium sulfate, CaSO4^.2H2O. It may be transparent to translucent when pure, but is often colored gray, yellow, red, brown or black by impurities. Gypsum has a white streak, is soft enough to be scratched by a fingernail (hardness of 2.0), and has a low specific gravity (2.2 to 2.4). Gypsum occurs in several forms, two of which are known to occur in the State. Selenite is a coarsely crystalline, transparent variety composed of flat, angular crystals that can be easily split apart.  Massive gypsum is a granular variety, showing no crystal form.
Occurrence: No commercial deposits are known to exist near the surface in Florida, though deposits of commercial extent are believed to be present at approximately 2,000 feet below the ground surface in Monroe County.
Use: When gypsum is heated and the water within it driven off, it forms a Plaster of Paris, which has the property of becoming hard after being mixed with water.