ONYX on’ icks, a banded semiprecious to precious stone which is a variety of chalcedony (q.v.), a very fine grained silica (silicon dioxide) and related to carnelian (q.v.) (cf. KJV and NEB Exod 28:20; Job 28:16). Like agate (q.v.) it consists of layers of different colors, in this case white and black, but the layers are even planes and the banding straight. It has been used largely in the making of cameos, as the design and background can be cut out so as to occur in differently colored layers. The chief localities are India and South America. Where the bands are white or bluish-white and red, or brownish red (sard—sardius, q.v.), the stone is referred to as sardonyx (q.v.)
The term onyx was applied by the Romans to a banded variety of marble—onyx marble. This consisted of concentric zones of calcite or aragonite (both calcium carbonate). Generally the bands are cream, yellow, buff, brown or red, due to impurities of iron oxide, but when pure it is white or translucent. This onyx marble was used for making ointment jars (Matt 26:7; Mark 14:3) and the banded variety of marble known as Algerian onyx was used in buildings of Carthage and Rome. Onyx marble can be scratched by a knife and is less precious than true onyx, which is much harder.
H. H. Read, Rutley’s Elements of Mineralogy, 26th ed. (1970), 274, 442, 443.