Adamant in classical mythology is an archaic form of diamond. The English word diamond is ultimately derived from adamas, via Late Latin diamas and Old French diamant. In ancient Greek ἀδάμας (adamas), genitive ἀδάμαντος (adamantos), literally 'unconquerable, untameable'. In those days, the qualities of hard metal (probably steel) were attributed to it, and adamant became an independent concept as a result.
In the Middle Ages adamant also became confused with the magnetic rock lodestone, and a folk etymology connected it with the Latin adamare, 'to love or be attached to'. Another connection was the belief that adamant (the diamond definition) could block the effects of a magnet. This was addressed in chapter III of Pseudodoxia Epidemica, for instance.
Since the contemporary word diamond is now used for the hardest gemstone, the increasingly archaic noun adamant has been reduced to mostly poetic or anachronistic use. In that capacity, the name, and various derivatives of it, are frequently used in modern media to refer to a variety of fictional substances.