adamant

(noun) very hard native crystalline carbon valued as a gem. Synonyms: diamond.

(adj) impervious to pleas, persuasion, requests, reason. Synonyms: adamantine, inexorable, intransigent. Examples: "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."