Vine's Expository Dictionary of NT Words
denotes "the sardian stone." Sardius is the word in the best texts in Revelation 4:3 (RV, "a sardius"), where it formed part of the symbolic appearance of the Lord on His throne, setting forth His glory and majesty in view of the judgment to follow. There are two special varieties, one a yellowish brown, the other a transparent red (like a cornelian). The beauty of the stone, its transparent brilliance, the high polish of which it is susceptible, made it a favorite among the ancients. It forms the sixth foundation of the wall of the heavenly Jerusalem, Revelation 21:20 .
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Vines, W. E., M. A. Entry for 'Sardius, Sardine'. Vine's Expository Dictionary of NT Words. https://www.studylight.org/dictionaries/eng/ved/s/sardius-sardine.html. 1940.