Sardonyx: a name compounded of sard and onyx , two precious stones, varieties of chalcedony or agate. The sardonyx combines the qualities of both, whence its name. It is mentioned only in (Revelation 21:20) The sardonyx consists of "a white opaque layer, superimposed upon a red transparent stratum of the true red sard." It is, like the sard, merely a variety of agate, and is frequently employed by engravers for signet-rings.