Gypsum is a naturally occurring rock, a metallic salt of calcium. It commonly forms as an evaporite from the dissolution of limestone by exposure to sulphuric acid from volcanic activity. Under certain conditions such as low-lying lagoons, continual cycles of dissolution and evaporation will agglomerate a “primary” deposit of gypsum.

Mineral gypsum formed is interspersed among other minerals. Primary deposit gypsums are characterized by a loose crystalline structure and high solubility in water. Over geologic time, gypsum from primary deposits can be dissolved by flowing water carried away in solution, forming a “secondary” deposit of a much purer gypsum. These secondary deposits form as beds of rock known as “massifs” that can be tens of feet thick and run for hundreds of feet. These massifs of gypsum rock are found all over the world and are the main source exploited as raw material for creating gypsum plaster.