Fausset's Bible Dictionary
shoham . Found in the land of Havilah (Genesis 2:12). Onyx means "nail"; then the agate, resembling in color a man's nail. Two onyx stones, with six names of Israel's tribes engraven on each, were on the high priest's shoulders as "stones of memorial unto Israel" (Exodus 28:9-12). The onyx was the second stone in the fourth row on his breast-plate (Exodus 28:20). Josephus (Ant. 3:7, section 5) calls the shoulder stones "sardonyxes" (compounded of sard or chalcedony and onyx, deep red and milkwhite layers alternating).
David's onyxes "prepared for the house of his God" (1 Chronicles 29:2) probably came from Tyre (Ezekiel 28:13). Tyre's king, like the high priest with his precious stones, was the type of humanity in its unfallen perfection in Eden; antichrist will usurp the divine King Priest's office (Zechariah 6:13; compare Acts 12:21-23). Job (Job 28:16) calls it "precious," but not so much so as "wisdom," priceless in worth. The Arabian sardonyxes have a black ground color, sachma , is Arabic "blackness"; opaque white covers black or blue strata. Sahara in Arabic means "to be pale"; from whence Gesenius derives shoham . The kinds of onyx and sardonyx vary so as to answer to either derivation. The onyx has two strata, the sardonyx has three.
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Fausset, Andrew R. Entry for 'Onyx'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/o/onyx.html. 1949.