Selenite is a form of Gypsum

 

Selenitesatin spardesert rose, and gypsum flower are four varieties of the mineral gypsum; all four varieties show obvious crystalline structure. The four "crystalline" varieties of gypsum are sometimes grouped together and called selenite.

All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no significant selenium; the similarity of names comes from both substances being named from the Ancient Greek word for the Moon.

 

Selenite is a transparent and colorless variety of the mineral gypsum.  It is known for being so soft that it can be scratched with a fingernail.  It has three unequal cleavages and is typically formed by evaporation of salt waters.  It has been found in crystals over 40 feet long in localities in Mexico.  Selenite is found in areas of Mexico and the U.S. in clusters of crystals resembling rosettes, popularly known as desert roses.

 

For more information try these links:

http://www.minerals-n-more.com/Gypsum_Info.html

http://giantcrystals.strahlen.org/america/naica.htm

http://www.vasichkominerals.com/ellsworth.htm