What does Bones mean when she says Booth has a perfect chromiun?

In the episode "The Goop on the Girl," Bones says that Booth has a "perfect chromium." The only definition I could find is "a lustrous, hard, brittle, metallic element used in alloy steels for hardness and corrosion resistance, as in stainless steel, and for plating other metals: chromium salts are used as pigments and mordants.." So what was she referring to? Maybe she meant Cranium? A little help if anyone has it.