Alabaster (sometimes called satin spar) is a name applied to varieties of two distinct minerals: gypsum (a hydrous sulfate of calcium) and the calcite (a carbonate of calcium). The former is the alabaster of the present day; the latter is generally the alabaster of the ancients.
The two kinds are readily distinguished from each other by their relative hardnesses. The gypsum kind is so soft as to be readily scratched by a finger-nail (hardness 1.5 to 2), while the calcite kind is too hard to be scratched in this way (hardness 3), though it does yield readily to a knife. Moreover, the calcite alabaster, being a carbonate, effervesces on being touched with hydrochloric acid, whereas the gypsum alabaster, when so treated, remains practically unaffected.

This substance, the "alabaster" of the Bible, is often termed Oriental alabaster, since the early examples came from the Far East. The Greek name alabastrites is said to be derived from the town of Alabastron, in Egypt, where the stone was quarried, but the locality probably owed its name to the mineral; the origin of the mineral-name is obscure, and it has been suggested that it may have had an Arabic origin. This "Oriental" alabaster was highly esteemed for making small perfume-bottles or ointment vases called alabastra, and this has been conjectured to be a possible source of the name. Alabaster was also employed in Egypt for canopic jars and various other sacred and sepulchral objects. A splendid sarcophagus, sculptured in a single block of translucent calcite alabaster from Alabastron, is in the Soane Museum, London. This was discovered by Giovanni Belzoni in 1817 in the tomb of Seti I near Thebes. It was purchased by Sir John Soane, having previously been offered to the British Museum.
When cut in thin sheets, alabaster is translucent enough to be used for small windows, and has been used so in medieval churches, especially in Italy. Large alabaster sheets are used extensively in the Cathedral of Our Lady of the Angels (dedicated 2002) of the Los Angeles (California) Archdiocese. The cathedral incorporates special cooling to prevent the panes from overheating and turning opaque.
Calcite alabaster is either a stalagmitic deposit, from the floor and walls of limestone caverns, or a kind of travertine, similarly deposited in springs of calcareous water. Its deposition in successive layers gives rise to the banded appearance that the marble often shows on cross-section, whence it is known as onyx-marble or alabaster-onyx, or sometimes simply as onyx — a term which should, however, be restricted to siliceous minerals. Egyptian alabaster has been extensively worked near Suez and near Assiut; there are many ancient quarries in the hills overlooking the plain of Tell el Amarna. The Algerian onyx-marble has been largely quarried in the province of Oran. In Mexico, there are famous deposits of a delicate green variety at La Pedrara, in the district of Tecali, near Puebla. Onyx-marble occurs also in the district of Tehuacán and at several localities in California, Arizona, Utah, Colorado and Virginia.