The mineral gypsum (calcium sulfate) occurs in several 'habits'.

These two specimens are selenite gypsum.

Specimens collected from Brewster County, Texas, are part of the Museum Mineral Education collection.

They are about 6" tall

Sand selenite

 Some times the Selenite crystals grow within sand bodies, making these remarkable, but fragile, collections of crystals

Specimen collected from west Texas, are part of the Museum Mineral Education collection.

Specimen: TMM TME (Education collection)
Mineral: Gypsum (calcium sulfate)
Form: Sand Selenite
Location: Hudspeth County, Texas
Collector: Glen L. Evans