What is the X in X-mas?
early 14c., from Old French diamant, from Medieval Latin diamantem (nominative diamas), from Vulgar Latin *adiamantem (altered by influence of the many Greek words in dia-), from Latin adamantem (nominative adamans) "the hardest metal," later, "diamond" (see adamant). Playing card suit is from 1590s; Sense in baseball is American English, 1875.
A form of pure carbon that occurs naturally as a clear, cubic crystal and is the hardest of all known minerals. It often occurs as octahedrons with rounded edges and curved surfaces. Diamond forms under conditions of extreme temperature and pressure and is most commonly found in volcanic breccias and in alluvial deposits. Poorly formed diamonds are used in abrasives and in industrial cutting tools.
(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.