adamant (adj.) Look up adamant at Dictionary.com
late 14c., "hard, unbreakable," from adamant (n.). Figurative sense of "unshakeable" first recorded 1670s. Related: Adamantly; adamance.
adamant (n.) Look up adamant at Dictionary.com
mid-14c., from Old French adamant and directly from Latin adamantem (nominative adamas) "adamant, hardest iron, steel," also figuratively, of character, from Greek adamas (genitive adamantos) "unbreakable, inflexible" metaphoric of anything unalterable, also the name of a hypothetical hardest material, perhaps literally "invincible," from a- "not" + daman "to conquer, to tame" (see tame (adj.)), or else a word of foreign origin altered to conform to Greek.

Applied in antiquity to white sapphire, magnet (perhaps via confusion with Latin adamare "to love passionately"), steel, emery stone, and especially diamond (see diamond). The word was in Old English as aðamans "a very hard stone."