Detail of an Assyrian relief of a winged genius (called an Apkallu), dated to the reign of Ashurnasirpal II (r. 883-859 BCE). Gypsum. Source: Christie’s.
Gypsum is an evaporite mineral, formed by the precipitation of saturated minerals from solution in arid climates, often in lagoon or endorheic lakes (those that drain a catchment but have no exits from their basins, such as the Great Salt Lake in the USA or Lake Turkana). Here one sees several ancient lake crusts, with needle like crystals that have grown between them, much like the copper sulphates crystals we all grew on strings as kids.
