When playing the classic game “20 Questions,” one may begin with the common opener: “Animal, vegetable, or mineral?”

For the ancient sea worm Perinereis cultrifera (which is still around today), the answer might not be so simple. Along with other predatory bristle worms, Perinereis cultrifera has jaws made from structural proteins and ions, which it uses for eating, crushing or biting. The unique makeup and properties of these jaws led some researchers to coin a new term to describe these types of materials: bio-metals, an emerging field of biophysical study.

The term “bio-metal” goes beyond identifiers like “metallike biomaterials” or “biomaterials with metallike properties,” which have been used in scientific literature to describe biomaterials with conductivity or strength values similar to metals. Instead, bio-metals can be categorized by three qualities: hardness, strain mechanics and ion-protein structure.