Adamantine
WordNet
adjective
(1) 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"
(2) Having the hardness of a diamond
(3) Consisting of or having the hardness of adamant