TOADSTONES: JEWELS FOUND IN THE HEAD OF TOADS

From ancient times through the Middle Ages up until 18th Century, it was thought that these stones were formed in the skull of toads. They were worn by people as amulets and rings to be applied as a magical antidote to poisons and evil. Actually, they are fossilized teeth of a fish, Lepidotes from the Jurassic and Cretaceous periods.

Toadstones are alluded to in Shakespeare's As You Like It (1599)

Sweet are the uses of adversity;
Which, like the toad, ugly and venomous,
Wears yet a precious jewel in his head.