adamantine

adjective

1. utterly unyielding or firm in attitude or opinion.
2. too hard to cut, break, or pierce.
3. like a diamond in luster.

adjective

1. very hard; unbreakable or unyielding
2. having the lustre of a diamond

adj.

c.1200, from Latin adamantinus “hard as steel, inflexible,” from Greek adamantinos, from adamas (see adamant (n.)).