(a.) Inflexible.

adamantine at a standstill cast-iron dour firm flintlike flinty frozen granitelike granitic grim hard hard-core immobile immotile immotive immovable immutable implacable impliable inductile inelastic inexorable inextensible inextensile inextensional inflexible intractable intractile intransigent iron irreconcilable irremovable irresilient lithic marblelike nonelastic nonstretchable obdurate pat petrified petrogenic relentless rigid rigorous rock rock-ribbed slaty standpat stationary steely stern stiff stone stubborn unaffected unalterable unbending unchangeable uncompromising unextendible unextensible unflexible ungiving unlimber unmalleable unmovable unmoved unmoving unpliable unpliant unrelenting unswayable untractable unyielding