Adamant (adjective)

1. Unyielding or inflexible in attitude or opinion.
2. Impossible to break, cut, or pierce.

Origin:

From middle english, from old french adamant, from latin adamantinus, from greek adamas "unconquerable, hard metal".

Examples:

1. He was adamant that he would never change his mind.
2. She was adamant that she would not go to the party.
3. The diamond is an adamant that can't be broken.
4. The adamant steel sword cut through the armor.
5. He was so adamant in his position that no one could change his mind.