ADAMANT

I. noun Etymology: Middle English, from Anglo-French, from Latin ~-, adamas hardest metal, diamond, from Greek Date: 14th century a stone (as a diamond) formerly believed to be of impenetrable hardness, an unbreakable or extremely hard substance, II. adjective Date: 1897 unshakable or insistent especially in maintaining a position or opinion ; unyielding , see: inflexible ~ly adverb