Alabaster is a name often applied to two distinct minerals. Calcite alabaster is a carbonate of calcium. It can form on the floors and walls of limestone caverns in stalagmitic deposits or form in the deposition of calcareous waters in springs or rivers, as a type of travertine. The travertine material is often beautifully colored and banded from impurities of iron found in the water. When used for decorative purposes, it is often referred to by trade-names, such as Oriental alabaster, onyx marble or Mexican onyx.

When the name alabaster is used without qualification, gypsum alabaster is usually the term that is being described. This is a hydrous sulfate of calcium and is a fine-grained variety of gypsum. This material occurs in nodular masses embedded in limestone. In its purest form, this material is snow-white. When associated with an oxide of iron, however, brown clouding and veining will occur in the stone. The coarser variety of alabaster is used to make what is commonly referred to as “plaster of Paris”. How can you tell the difference?

Calcite alabaster (hardness 3) is harder than gypsum alabaster. This means that the gypsum kind is so soft that it can easily be scratched with your finger-nail (hardness 1.5 to 2). In addition, because the calcite alabaster is a carbonate, it will effervesce when it comes in contact with hydrochloric acid. The gypsum alabaster will remain virtually unaffected. The gypsum variety is also soluble in water making it impractical for use outdoors. Both forms are easily carved and therefore are often used for ornamental architectural detail work and decorative items.