Cal. Penal Code § 16405

As used in this part, “composite knuckles” means any device or instrument made wholly or partially of composite materials, other than a medically prescribed prosthetic, that is not metal knuckles, that is worn for purposes of offense or defense in or on the hand, and that either protects the wearer’s hand while striking a blow or increases the force of impact from the blow or injury to the individual receiving the blow.