Desert rose of gypsum crystals. Close-up of petal-shaped crystal aggregrates of tabular gypsum (hydrated calcium sulphate), which form in arid regions such as the Sahara Desert. The process is a result of early mineral diagenesis (formation of rock from a sediment), especially in salt flats where water evaporation rates are high. The crystals commonly incorporate aeolian (wind-blown) sand grains which contribute to the mineral colour and give it a sandy crystalline texture. This sample was found in the Sahara Desert, in Tunisia.