Wiktionary
bone hard
a. As hard as a bone, rock hard.
Usage examples of "bone hard".
Holding the Bone hard against my chest, I still felt the heat of the Shoes, but only distantly, as if they were somehow much further off.
She caught it with her blade, ripping through something hard - bone hard.
Bone thin and bone hard he was, with a face that might have been chipped from flint.