As other nodes about Gypsum have pointed out, it is one of the most common minerals on earth (CaSO4+2H2O). There are thousands of pounds of it in the walls of a typical house, and huge deposits of it, both natural and man-made. The natural deposits are as a result of drying up of seabeds, but the man-made ones are a result of man's efforts to control air pollution. Because it is somewhat alkaline, with a ph of about 11, it can also be classified as low level hazardous waste in an industrial setting. Huge piles of gypsum have been built up over the years as a byproduct of a process known as scrubbing, which reduces pollution by removing sulfates from smokestack emissions caused by burning coal. Using lime (CaOH), the sulphate emissions are precipitated out. Commonly, these emissions are in the form of sulfuric acid. The Calcium chemically binds to the sulphate ions, forming gypsum, which precipitates out of the smokestack scrubbers as a relatively harmless powder, rather than the corrosive vapor that is sulfuric acid, which poisons lakes, causes rust and corrosion, and God knows what else.
I discovered these facts when reading an article about a controversy caused by a proposed highway being built on manmade deposits of gypsum that were piled along the Mississippi River. The United States EPA raised objections to the plan, due to the legalities of dealing with hazardous waste.