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