Merriam-Webster's Collegiate® Dictionary, 11th Edition
ADAMANT
I. noun
Etymology: Middle English, from Anglo-French, from Latin adamant-, 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 adamantly adverb