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 (see CARNELIAN) . Most of the modern sardonyx is cut from South American agate, modified in colour by artificial treatment .