adamant adamantine cast-iron changeless constant dour firm fixed flinty grim hard hard-core immovable immutable implacable impliable inalterable incommutable inconvertible indefeasible inductile inelastic inert inexorable inextensible inextensile inextensional inflexible insusceptible of change intractable intractile intransigent intransmutable invariable iron irreconcilable irresilient irretrievable irreversible irrevocable lasting noble nonelastic nonreturnable nonreversible nonstretchable permanent relentless reverseless rigid rigorous rock-ribbed steely stern stiff stubborn unaffected unalterative unaltered unbending unchangeable unchanged unchanging uncompromising undeflectable undeviating unextendible unextensible unflexible ungiving unlimber unmalleable unmodifiable unmovable unmoved unpliable unpliant unrelenting unremitting unrestorable unreturnable unsusceptible untractable unvariable unvarying unyielding