ad·a·mant 
(ădə-mənt, -mănt′)
(ădə-mənt, -mănt′)adj.
Not willing to change one's opinion, purpose, or principles; unyielding.
n.
1. A stone once believed to be impenetrable in its hardness.
2. An extremely hard substance.
[From Middle English, a hard precious stone, from Old French adamaunt, from Latin adamās, adamant-, from Greek, unconquerable, hard steel, diamond; see demə- in the Appendix of Indo-European roots.]
ada·mance, ada·man·cy n.
ada·mant·ly adv.