adamant
(ˈædəmənt) adjective 1 unshakable in purpose, determination, or opinion; unyielding
▷ noun 3 any extremely hard or apparently unbreakable substance
4 a legendary stone said to be impenetrable, often identified with the diamond or loadstone
[Old English: from Latin adamant-, stem of adamas, from Greek; literal meaning perhaps: unconquerable, from a-1 + daman to tame, conquer]
> ˈadamantly adverb