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.