Selenite (CaSO4ยท2H2O) is a common mineral in sedimentary
environments so I see a lot of it when looking
fossils. Enormous crytals have been found in
a cave in Mexico

This mineral is often called, "Gypsum,"
and is the source of Plaster of Paris.

It is the standard for hardness 2 on the Mohs scale.
It cleaves readily and thin pieces can be bent.

Single crytals—selenite forms monoclinic crystals.

These are twinned crystals.

This crystal group is often referred to as a selenite rose.