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