SARDONYX, an ornamental- stone much used for seals and cameos. It usually consists of a layer of sard or carnelian with one of milk-white chalcedony, but it may present several alternating layers of these minerals. The sardonyx is therefore simply an onyx in which some of the bands are of sard or carnelian: if, however, the latter is present the stone is more appropriately called a "carnelian onyx."
It was considered by ancient authorities that a fine Oriental sardonyx should have at least three strata—a black base, a white intermediate zone and a superficial layer of brown or red; these colours typifying the three cardinal virtues—humility (black), chastity (white) and modesty or martyrdom (red). The ancients obtained sardonyx from India, and the Indian locality, Mount Sardonyx, referred to by Ptolemy, is supposed to have been near Broach, where agates and carnelians are still worked. In the Revised Version of the Old Testament, Ex. xxviii. 18, " sardonyx " is given in the margin as an alternative reading for " diamond," the word by which the Hebrew yahalom is usually translated. The stone known to the Romans as aegyptilla may have been a kind of sardonyx, or perhaps a nicolo, which is an onyx with a thin translucent milky layer on the surface.
Imitations of sardonyx have been made by cementing together two or three stones of the required colours, while baser counterfeits have been produced in paste. By coating a sard or carnelian with sodium carbonate and then placing the stone on a red-hot iron a white layer may be produced, so that a kind of sardonyx is obtained.
Most of the modern sardonyx is cut from South American agate, modified in colour by artificial treatment. GemWerx sardonyx comes from the famous Agate Creek deposits of North Queensland and is totally natural and untreated.