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 it 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 define alabaster only as the gypsum is a hydrous, while calcite
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 marked with patterns of swirling bands of cream and brown". "Onyx-marble" is a traditional, but geologically inaccurate, name because both onyx have geological definitions that are distinct from even the broadest definition of "alabaster".
In general, ancient alabaster is calcite in the wider Middle East, while it is gypsum. 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 when treated with hydrochloric acid , while gypsum alabaster remains almost unaffected.