Chemistry:  CaSO4.2 (H2O) - Hydrated Calcium Sulphate
Selenite, satin spar, desert rose, and gypsum flower are the four crystalline varieties of gypsum. All gypsum varieties are very soft mineral (hardness: 2 on Mohs Scale) composed of calcium sulfate dihydrate (meaning has two molecules of water), with the chemical formula CaSO₄·2H₂O. Selenite can be scratched with a fingernail.

The four crystalline varieties of gypsum are sometimes grouped together; however, selenite, satin spar, desert rose, and gypsum flower have differences.
The most important identifying characteristic is how soft gypsum is. Also because gypsum has natural insulating properties, it feels warm to the touch.
General identifying descriptions of the related crystalline varieties are:
 Selenite

• most often transparent and colorless
• if selenite crystals show translucency, opacity, and/or color, it is caused by the presence of other minerals (including druse)
• druse is the crust of tiny, minute, or micro crystals that form or fuse either within or upon the surface of a rock cavity, geode, or another crystal