Alabaster is a mineral or rock that is soft and often used for carving, as well as being processed for plaster powder. The term is used in different ways by archaeologists and the stone processing industry on the one hand, and geologists on the other. The first use is in a wider meaning, covering varieties of two different minerals: the fine-grained massive type of gypsum, as well as the fine-grained banded type of calcite. Geologists only define the gypsum variety as alabaster. Chemically, gypsum is a hydrous sulfate of calcium, while calcite is a carbonate of calcium.
Both types of alabaster have broadly similar properties. They are usually light-coloured, translucent and soft stones that have been used throughout human history mainly for carving decorative artifacts.
The calcite variety is also known as onyx-marble, Egyptian alabaster, or 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" must be understood as a traditional, but geologically inaccurate term, since both onyx and marble have geological definitions distinct from even the widest one applicable for alabaster.
In general (but not always), 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 water-soluble. They have been used for making a variety of indoor artworks and carvings, as they will not survive long outdoors.
The two kinds are readily distinguished by differences in their hardness: gypsum alabaster is so soft it can be scratched with a fingernail (Mohs hardness 1.5 to 2), while calcite cannot be scratched in this way (Mohs hardness 3), although it does yield to a knife. Moreover, calcite alabaster, being a carbonate, effervesces when treated with hydrochloric acid, while gypsum alabaster remains nearly unaffected when thus treated....LESS