(Greek alabastros, -on; Latin alabaster, -trum; of uncertain origin).
The substance commonly known as alabaster is a fine-grained variety of gypsum (calcium sulphate) much used for vases and other ornamental articles. Oriental alabaster, the alabastrites of the classical writers, is a translucent marble (calcium carbonate) obtained from stalagmitic deposits; because of its usually banded structure, which gives it some resemblance to onyx, it is also called onyx marble, or simply, though incorrectly, onyx. From remote times it was highly esteemed for decorative purposes. Among the ancients Oriental alabaster was frequently used for vases to hold unguents, in the belief that it preserved them; whence the vases were called alabasters, even when made of other materials. Such was the "alabastrum unguenti" (Matthew 26:7; Mark 14:3, Luke 7:37), with which the sinful woman anointed the Saviour. The vase, however, though probably of alabaster, was not necessarily of that material, as our English translation "alabaster box of ointments" seems to imply.
THOMAS in VIG., Dict. de la Bible, I, 330.