adamantine (adj.)

"made of adamant; having the qualities of adamant" (hard, unyielding, unbreakable, inflexible), c. 1200, from Latin adamantinus "hard as steel, inflexible," from Greek adamantinos "hard as adamant," from adamas (genitive adamantos) "unbreakable, inflexible," as a noun, "hardest material" (see adamant (n.)).