Gypsum is a source of Plaster of Paris and other plasters. It is a very soft mineral. It is a pigment, and was found in Tutankhamun's paint-box. The rose type formation is called Desert Rose. Gypsum dissolves in water, and so such crystals, which take a long time to grow, only happen in very dry conditions, such as deserts.
Selenite is the same chemical compound as gypsum. It was anciently supposed to wax and wane with the moon. This was probably because it used to drop out of any jewelry holding it. They thought this was because it got bigger and smaller, but in fact, selenite is very soft, and so would be worn away by any metal gripping it.
Gypsum - Satin Spar
Gypsum from the cliffs in Penarth, near Cardiff
Cluster of Selenite crystals
A single Selenite crystal