adamant

n. [ OE. adamaunt, adamant, diamond, magnet, OF. adamant, L. adamas, adamantis, the hardest metal, fr. Gr. 'ada`mas, -antos; 'a priv. + dama^, n to tame, subdue. In OE., from confusion with L. adamare to love, be attached to, the word meant also magnet, as in OF. and LL. See Diamond, Tame. ] 1. A stone imagined by some to be of impenetrable hardness; a name given to the diamond and other substances of extreme hardness; but in modern mineralogy it has no technical signification. It is now a rhetorical or poetical name for the embodiment of impenetrable hardness. [ 1913 Webster ]

Opposed the rocky orb
Of tenfold adamant, his ample shield. Milton. [ 1913 Webster ]

2. Lodestone; magnet. [ Obs. ] “A great adamant of acquaintance.” Bacon. [ 1913 Webster ]

As true to thee as steel to adamant. Greene. [ 1913 Webster ]