Which mineral is widely used to make plaster of Paris?

Gypsum is the mineral widely used to make plaster of Paris.

Gypsum is hydrated calcium sulfate, with the chemical formula CaSO₄·2H₂O. When gypsum is heated, it loses part of its crystal water and becomes calcium sulfate hemihydrate, the powder commonly called plaster of Paris. Mixing that powder with water causes it to rehydrate and harden.

The mineral commonly forms when saline water evaporates, making it an evaporite. It can occur in clear crystals called selenite, fibrous forms called satin spar, and fine-grained masses called alabaster. These are varieties or forms of gypsum rather than entirely different minerals.

Gypsum has a Mohs hardness of 2, so a fingernail can usually scratch it. It is used in wallboard, cement, soil conditioners, casts, and molds. Plaster of Paris is therefore a processed gypsum product, not the mineral’s ordinary name.