ALABASTER, in present-day usage refers to a very fine grained and compact, snow-white or light-colored, massive variety of gypsum (hydrated calcium sulphate) which is used for interior ornamental work. Alabaster of the ancients, or oriental alabaster (including Algerian onyx), was marble. It was used for making ointment jars (Matt 26:7; Mark 14:3) and the Algerian onyx was used in buildings of Carthage and Rome. The marble is generally banded with the so-called onyx-marbles consisting of concentric zones of calcite or aragonite (both calcium carbonate). When pure it is white or translucent. Impurities give a great variety of colors, particularly cream, yellow, buff, brown and red due to the presence of iron oxide. Strictly speaking it is neither marble nor onyx, for the onyx is a banded chalcedony (q.v.) composed largely of silicon dioxide.

Formation is the result of deposition from cold water solutions in caves (q.v.), particularly as stalactites and stalagmites, in crevices and around the exits of springs. This type of deposition occurs in regions with limestone (calcium carbonate) strata. The calcium carbonate is dissolved by ground water containing carbon dioxide which moves along bedding planes and joints. In caves, formed just below the water table, calcite or aragonite is precipitated when carbon dioxide is lost from the ground water. Stalactites hang vertically from the cave roof with concentric layers of crystals being added as ground water slowly flows down the outer surface. Rate of growth generally is of the order of 0.25-3mm/year. Concentrically banded stalagmites grow upward from the cave floor toward the drip surface in the roof.

Bibliography R. W. Fairbridge (ed.), The Encyclopedia of Geomorphology (1968), 652, 653, 1048-1052; H. H. Read, Rutley’s Elements of Mineralogy, 26th ed. (1970), 270-277, 280-282.