adamantine

[ad-uh-man-teen, -tin, -tahyn] /ˌæd əˈmæn tin, -tɪn, -taɪn/
adjective
1.
utterly unyielding or firm in attitude or opinion.
2.
too hard to cut, break, or pierce.
3.
like a diamond in luster.
Origin
1200-1250; Middle English < Latin adamantinus < Greek adamántinos. See adamant, -ine1
Examples from the web for adamantine
  • In doing so, it continues to exert pressure on the adamantine surface of oppression.
  • Her fluent mendacity, combined with adamantine confidence, made her really indomitable.
  • adamantine adherence to political principle is nothing new, of course.
  • The unambiguous factuality of a single achievement is adamantine.
  • Diamond is the hardest of all naturally occurring substances, is transparent if pure, and has an adamantine luster.
  • The color is transparent emerald green with a green streak and adamantine luster.
  • Light was provided by adamantine candles and/or coal or lard oil lamps.
  • Most diamonds are transparent and with a strong adamantine luster.
British Dictionary definitions for adamantine

adamantine

/ˌædəˈmæntaɪn/
adjective
1.
very hard; unbreakable or unyielding
2.
having the lustre of a diamond
Word Origin and History for adamantine
adj.

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