In his ancient list of "white gems," Isidore of Seville mentions the "cinaedia," which he describes as follows:
Cinaedia is found in the brain of a fish with the same name; it is white and oblong. People say that with it they predict calm or storm at sea (Etymologies 16.10.8)
This seems to match the description of a lucky stone, except that freshwater drums (Aplodinotus grunniens) would hardly have been endemic to Isidore's (Old World) setting. What fish could Isidore have been referring to?