adamant

Short definition from Wiktionary

Adjective

1. 1
   Firm; unshakeable; unyielding; determined.
2. 2
   Very difficult to break, pierce, or cut.

Noun

1. 1
   An imaginary rock or mineral of impenetrable hardness; a name given to the diamond and other substances of extreme hardness.
2. 2
   An embodiment of impregnable hardness.
3. 3
   A lodestone.
4. 4
   A substance that neutralizes lodestones.