Alabaster is a form of Gypsum

Alabaster is a name that has been used for two different materials: gypsum (a hydrous sulfate of calcium) and calcite (a carbonate of calcium). Gypsum is what we call alabaster today; calcite is the alabaster of the ancients. Both are easy to work, with an attractive appearance, and have been used for making a variety of artworks and objects, especially small carvings.

The two minerals are easy to tell apart because of differences in their hardness. Gypsum is so soft that it can be scratched with a fingernail (Mohs hardness 1.5 to 2), while calcite is too hard to be scratched in this way (Mohs hardness 3), although it does scratch easily with a knife. Another way to tell them apart uses an acid. The calcite alabaster, being a carbonate, fizzes when touched by acids. Whereas the gypsum alabaster, when treated with acid, remains practically unaffected.

Due to the characteristic color of white alabaster, the term has entered the vernacular as a phrase for white things, particularly "alabaster skin", which means very light and quite transparent, and the familiar phrase "alabaster citys" in the song "America the Beautiful".

 

More information is available at:

http://stoneplus.cst.cmich.edu/alabaster.htm

http://en.wikipedia.org/wiki/Alabaster

http://www.mineralszone.com/gemstones/alabaster.html

http://www.mindat.org/min-90.html