Smith’s Bible Dictionary
Diamond: (Heb. yahalom), a gem crystallized carbon, the most valued and brilliant of precious stones, remarkable for its hardness, the third precious stone in the second row on the breastplate of the high priest, (Exodus 28:18; 39:11) and mentioned by Ezekiel, (Ezekiel 28:13) among the precious stones of the king of Tyre. Some suppose yahalom to be the "emerald." Respecting shamir , which is translated "Diamond" in (Jeremiah 17:1) see under ADAMANT.