Patinated Copper, Sterling silver, Pure Silver
Crustaceans have a sort of evolutionary arms race between the sexes, where females have grown chitinous coverings over their sex organs, and males have developed increasingly sharp claws and spears to pierce them in mating.
Coupling in many insects is a violent affair, where only an aggressive, powerful male will pierce the armor of the resistant female. Those with the sharpest penises or best armored vaginas are also the strongest and healthiest of the species, and thus ideal mates.