Alabaster is a mineral or rock that is soft, often used for carving, and is processed for plaster powder. Archaeologists and the stone processing industry use the word differently from geologists. The former use is in a wider sense that includes varieties of two different minerals: the fine-grained massive type of gypsum and the fine-grained banded type of calcite. Geologists define alabaster only as the gypsum type. Chemically, gypsum is a hydrous sulfate of calcium, while calcite is a carbonate of calcium.

Both types of alabaster have similar properties. They are usually lightly colored, translucent, and soft stones. They have been used throughout history primarily for carving decorative artifacts.

The calcite type is also denominated “onyx-marble”, “Egyptian alabaster”, and “Oriental alabaster” and is geologically described as either a compact banded travertine or “a stalagmitic limestone marked with patterns of swirling bands of cream and brown”.”Onyx-marble” is a traditional, but geologically inaccurate, name because both onyx and marble have geological definitions that are distinct from even the broadest definition of “alabaster”.

Alabaster bust (not the head) of Septimius Severus at the Musei Capitolini, Rome

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 probably 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, and they will not survive long outdoors.

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