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Fausset's Bible Dictionary
Hebrew kadkod , from kadad , "to sparkle". The "windows" being of this gem (Isaiah 54:12) implies transparency. Gesenius thinks the ruby or carbuncle is meant. It was imported from Syria to Tyre (Ezekiel 27:16). Hebrew Sheba (from Sheba whence it came to Tyre), Exodus 28:19; Exodus 39:12, is rightly translated "agate," a semi-transparent uncrystallized quartz, mainly silica, with concentric layers of various tints; the second stone of the third row on the high priest's breast-plate. The English term is drawn from that of the Greeks, who found agate in the river Acheres, in Sicily, and hence named it.
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Fausset, Andrew R. Entry for 'Agate'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/agate.html. 1949.