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