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late 14c., "hard, unbreakable," from adamant (n.). Figurative sense of "unshakeable" first recorded 1670s. Related: Adamantly; adamance.
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."
(Heb. shamir), Ezek. 3:9. The Greek word adamas means diamond. This stone is not referred to, but corundum or some kind of hard steel. It is an emblem of firmness in resisting adversaries of the truth (Zech. 7:12), and of hard-heartedness against the truth (Jer. 17:1).