King James Bible Dictionary

(Revelation 21:20), a species of the carnelian combining the sard and the onyx, having three layers of opaque spots or stripes on a transparent red basis. Like the sardine, it is a variety of the chalcedony.

A precious stone in the foundation of the heavenly city.
Revelation 21:20

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.

S'ARDONYX, noun [Latin sardonyches, from Gr. from Sardis, a city of Asia Minor, and a nail; so named, according to Pliny, from the resemblance of its color to the flesh under the nail.]

A silicious stone or gem, nearly allied to carnelian. Its color is a reddish yellow, or nearly orange. We are informed that the yellow or orange colored agate, with an undulating surface, is now often called sardonyx