Английский Этимологический словарь - adamant (adj.)

- 1387, "hard, unbreakable," from n. (1345), from L. adamantem, from Gk. adamas (gen. adamantos) "unbreakable," the name of a hypothetical hardest material. Perhaps from a- "not" + damnanai "conquer," or of foreign origin. Applied to white sapphire, magnet, and especially diamond. Extended sense first recorded 1677.