Diamond

(1.) A precious gem (Heb. yahalom', in allusion to its
hardness), otherwise unknown, the sixth, i.e., the third in the
second row, in the breastplate of the high priest, with the name
of Naphtali engraven on it (Ex. 28:18; 39:11; R.V. marg.,
"sardonyx.")

(2.) A precious stone (Heb. shamir', a sharp point) mentioned
in Jer. 17:1. From its hardness it was used for cutting and
perforating other minerals. It is rendered "adamant" (q.v.) in
Ezek. 3:9, Zech. 7:12. It is the hardest and most valuable of
precious stones.