How Is Alabaster Rock Formed?

Calcite Alabaster

• One way that rock is formed is by decomposition. Calcite alabaster, a carbonate rock, is part of the limestone rock variety and is formed by the decomposition of the exoskeletons of sea life, such as coral, clams or snails. As the sea creatures die, their shells decompose and become part of the rock. Calcite alabaster, although no longer used today, was found in stalagmite deposits in limestone caverns or in springs.

Gypsum Alabaster

• Gypsum alabaster, the type of alabaster common today, is a sedimentary rock formed by evaporation. Gypsum alabaster is formed by chemical precipitation, specifically an increase in the salinity of a body of water. As the salt content of a body of water rises and the concentration of gypsum and halite, or rock salt, increases, gypsum alabaster is precipitated. Gypsum alabaster is found in massive beds and quarries of rock gypsum.

Differences between Calcite and Gypsum Alabaster

• Calcite and gypsum alabaster are quite different materials used in different time periods. The varieties of alabaster are formed in different ways, with one formed of decomposed material and the other by evaporation and precipitation. Calcite alabaster is a harder rock that stands up well to pressure. Gypsum alabaster is soft and chalky. If one were to run a finger over a piece of gypsum alabaster, a chalky residue would be left on the finger.

Uses of Calcite and Gypsum Alabaster

• In ancient Egypt, calcite alabaster was carved to make many different things such as sphinxes, sarcophagi, canopic jars and other varieties of stone work. In the Art Deco and Art Nouveau periods, gypsum alabaster was used to diffuse light in lamps and lamp shades because of its translucency. Alabaster made a comeback in the 1980s as the trends shifted to Art Deco-inspired art, architecture and furnishings. Gypsum alabaster is used in plaster and in plaster of Paris.