Synonyms for Adamant (same or very similar meaning)
WordNet sense 1 (very hard native crystalline carbon valued as a gem):
diamondWordNet sense 2 (impervious to pleas, persuasion, requests, reason):
adamantine, inexorable, intransigentWordNet sense 3 (incapable of change):
inflexibleFrom the ODE community, based on WordNetadd/edit