Gypsum is a common sulfate mineral made of calcium sulfate. Gypsum has been used as a building material for a long time, possibly since the neolithic. Today, it is an ingredient of plaster. It can also be used as a fertilizer. It is softer than most minerals, with a value of 2 on the Mohs scale of mineral hardness. This mineral can be, somewhat, scratched with your fingernail. A fingernail, with a value of 2.5 on the Mohs scale of mineral hardness is in fact higher than the mineral shown above.