Alabaster is a name applied to varieties of two distinct minerals, when used as a material: gypsum (a hydrous sulfate of calcium) and calcite (a carbonate of calcium). The latter is the alabaster of the present day; generally, the former 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 kinds are distinguished from one another readily, because of differences in their relative hardness. The gypsum kind is so soft as to be readily scratched with a fingernail (Mohs hardness 1.5 to 2), while the calcite kind is too hard to be scratched in this way (Mohs hardness 3), although it does yield readily to a knife. Moreover, the calcite alabaster, being a carbonate, effervesces upon being touched with hydrochloric acid, whereas the gypsum alabaster, when thus treated, remains practically unaffected.
Due to the characteristic color of white alabaster, the term has entered the vernacular as a metonym for white things, particularly "alabaster skin", which means very light and quite transparent, and possibly derives from the use of alabaster for tomb effigies.