adamant

Concise Oxford English Dictionary © 2008 Oxford University Press:
adamant/ˈadəmənt/

▶adjective refusing to be persuaded or to change one's mind. ▶noun archaic a legendary rock or mineral associated at times with both diamond and lodestones.

– derivatives
adamance noun,
adamancy noun,
adamantly adverb.

– origin OE (as n.), from OFr. adamaunt-, via L. from Gk adamas, adamant-, ‘untameable, invincible’ (later used to denote the hardest metal or stone, hence diamond), from a- ‘not’ + daman ‘to tame’.