Synonyms and related words :
bony, cement, cemental, concrete, dense, diamondlike, dure, flintlike, flinty, granitelike, granitic, hard, hard as nails, hardhearted, horny, iron-hard, ironlike, lapideous, lithoid, lithoidal, marble, marblelike, obdurate, osseous, resistant, resistive, rocklike, rocky, solid, steellike, steely, stonelike, stony, tough,