Answer: adamantine

AnswerCrossword Clue
ADAMANTINEImpenetrable.
ADAMANTINEImmovable.
ADAMANTINEUnyielding.
ADAMANTINEFirm.
ADAMANTINEUnyielding
ADAMANTINEHard
adamantineof adamant; hard as adamant also ADAMANTEAN
adamantineutterly unyielding or firm in attitude or opinion.
adamantineUnbreakable