Adamance

utterly unyielding in attitude or opinion in spite of all appeals, urgings, etc.
too hard to cut, break, or pierce.
any impenetrably or unyieldingly hard substance.
a legendary stone of impenetrable hardness, formerly sometimes identified with the diamond.
adjective
unshakable in purpose, determination, or opinion; unyielding
a less common word for adamantine (sense 1)
noun
any extremely hard or apparently unbreakable substance
a legendary stone said to be impenetrable, often identified with the diamond or loadstone
adj.

late 14c., “hard, unbreakable,” from adamant (n.). Figurative sense of “unshakeable” first recorded 1670s. Related: Adamantly; adamance.
n.

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).