(σαρδόνυξ , from σάρδιον, the sardius, and ὄνυξ, the onyx) is mentioned in the New Test. once only — viz. in Revelation 21:20 — as the stone which garnished the fifth foundation of the wall of the heavenly Jerusalem. "By sardonyx," says Pliny (N.H. 37, 6), who describes several varieties, "was formerly understood, as its name implies, a sard with a white ground beneath it, like the flesh under the fingernail." The sardonyx consists of "a white opaque layer, superimposed upon a red transparent stratum of the true red sard" (King, Ant. Gems, p. 9). It is, like the sard, merely a variety of agate, and is frequently employed by engravers for the purposes of a signet ring. It is a species of onyx, distinguished from the common stone of that name by having its different colors, red and white, disposed in alternate bands. But there is another stone so called, whose tint is reddish yellow or orange, with sometimes a tinge of brown (Moore, Anc. Mineral. p. 153).