Hastings' Dictionary of the Bible (1898)
SAR'DONYX, only mentioned in Rev 21:20. Like the sardine, this stone is a variety of chalcedony. The sardonyx combines the qualities of the sard and onyx, whence its name. In this gem as used, a white opaque layer rests upon a transparent red stratum, as King states, or the reverse, according to Pliny. The sard and sardonyx are found in Judaea.