adamantine
adjective
definition 1: made of, resembling, or having the hardness of adamant*.
definition 2: firmly decided or fixed; unyielding; adamant.
example: It is useless to plead with someone known for his adamantine will.
Related word:
*adamant
adjective
unlikely to change in response to any request or argument; firmly decided or fixed; unyielding.
He was adamant in his refusal to join any political party.
noun
1. a legendary stone of great hardness.
2. any extremely hard, unbreakable, or impenetrable substance.