Vine's Expository Dictionary of NT Words
a name which indicates the formation of the gem, a layer of sard, and a layer of onyx, marked by the red of the sard and the white of the onyx. It was used among the Romans both for cameos and for signets. It forms the fifth foundation of the wall of the heavenly Jerusalem, Revelation 21:20 .
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Vines, W. E., M. A. Entry for 'Sardonyx'. Vine's Expository Dictionary of NT Words. https://www.studylight.org/dictionaries/eng/ved/s/sardonyx.html. 1940.