Adamant in classical mythology is an archaic form of diamond. In fact, the English word diamond is ultimately derived from adamas, via Late Latin diamas and Old French diamant. In ancient Greek ἀδάμας, genitive ἀδάμαντος, literally "unconquerable, untameable". In those days, the qualities of hard metal were attributed to it, and it "adamant" became as a result an independent concept.
