One of the interesting ingredients of animal origin preserved at the History of Pharmacy Collection in Cluj-Napoca is crab's eyes (Lapides Cancrorum or Oculi Cancrorum). The strange product was in fact used as a natural source of calcium carbonate (for calcium supplements and against stomach hyper acidity) besides oyster and mussel shells, pincers, and coral. The term designating them stems from the confusion of their aspect, resembling an eye, but they are calcareous concretions found in the head, or rather stomach, of river crabs. They are generally about the size of peas, or larger and have a laminated inner structure (through which they can be distinguished from fakes).
Read more on crab stones as pat of materia medica in German HERE and in Polish HERE and on how crabs produce these gastroliths - in English HERE.