adamant

adamant is a Noun

Definition:

[1] A stone imagined by some to be of impenetrable hardness; a name given to the diamond and other substance of extreme hardness; but in modern minerology it has no technical signification. It is now a rhetorical or poetical name for the embodiment of impenetrable hardness. Opposed the rocky orb Of tenfold adamant, his ample shield. Milton.

[2] Lodestone; magnet. [Obs.] "A great adamant of acquaintance." Bacon. As true to thee as steel to adamant. Greene.