Pencil Selenite is a form of Gypsum
Pencil Selenite (or Pencil Gypsum) is long narrow crystals of gypsum found only in the old gypsum mine in Grand Rapids, Michigan.
Selenite, satin spar, desert rose, and gypsum flower are four varieties of the mineral gypsum; all four varieties show obvious crystallinestructure. 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.
For more information about other types of Selenite and other forms of Gypsum try these links: