Alabaster

Alabaster is a mineral and a soft rock used for carvings and as a source of plaster powder. Archaeologists, geologists, and the stone industry have different definitions and usages for the word alabaster. In archaeology, the term alabaster is a category of objects and artefacts made from the varieties of two different minerals: (i) the fine-grained, massive type of gypsum, and (ii) the fine-grained, banded type of calcite.

Chemically, gypsum is a hydrous sulfate of calcium, whereas calcite is a carbonate of calcium. As types of alabaster, gypsum and calcite have similar properties, such as light color, translucence, and soft stones that can be carved and sculpted; thus the historical use and application of alabaster for the production of carved, decorative artefacts and objets d’art. Calcite alabaster also is known as: onyx-marble, Egyptian alabaster, and Oriental alabaster, which terms usually describe either a compact, banded travertine stone or a stalagmitic limestone colored with swirling bands of cream and brown.

In general, ancient alabaster is calcite in the wider Middle East, including Egypt and Mesopotamia, while it is gypsum in medieval Europe. Modern alabaster is most likely calcite but may be either. Both are easy to work and slightly soluble in water. They have been used for making a variety of indoor artwork and carving, as they will not survive long outdoors.

The two kinds are readily distinguished by their different hardnesses: gypsum alabaster (Mohs hardness 1.5 to 2) is so soft that a fingernail scratches it, while calcite (Mohs hardness 3) cannot be scratched in this way but yields to a knife. Moreover, calcite alabaster, being a carbonate, effervesces when treated with hydrochloric acid, while gypsum alabaster remains almost unaffected.