adamant

nouns

very hard native crystalline carbon valued as a gem
adamant, diamond

adjectives

impervious to pleas, persuasion, requests, reason
"he is adamant in his refusal to change his mind"; "Cynthia was inexorable; she would have none of him"- W.Churchill; "an intransigent conservative opposed to every liberal tendency"
adamant, adamantine, inexorable, intransigent
is derived from
adamance