I am often asked, "Shane, how can I condition my knuckles?" or harden the bones in your hand, or strengthen your wrist, or some variation of this. In this FAQ video I talk about why I don't believe in knuckle conditioning, and why you should stick to forearm strengthening exercises instead.
Don't injure your hands. You don't need to strengthen your knuckles for martial arts or harden them in some crazy way like punching a wall. Your fists and hands will naturally condition when hitting a heavy bag (with handwraps and gloves on!), plus other forearm workouts. Don't cause unnecessary harm to your hands trying to condition them.