By the way, the process which Ramsey's power forms a natural defense is as follows. First his skin hardens, second his flesh, and muscle increase in density. And third his blood surrounds the bone to keep his arm attached as much as possible. The effectiveness depends on the opponent, someone using advanced tech, or a super like Icon, wielding any weapon would cause harm. Though it does depend on the weapon. Something like a Katana would be ineffective at cutting his skin. If used with brute force alone, but an Axe would be far more effective. Anything like getting hit by a bat by some street thug, or glass jabbed into his skin would normally break, but it really all depends. 

Ramsey isn't invulnerable any high caliber bullet, or explosive would do a lot of damage, but a simple knife wouldn't do anything. Except bend and break.