Toadstone

The toadstone, also known as bufonite, is a mythical stone or gem that was thought to be found in the head of a toad. It was supposed to be an antidote to poison and in this it is not unlike Batrachite, supposedly formed in the heads of frogs. Toadstones were actually the button-like fossilized teeth of Lepidotes, an extinct genus of ray-finned fish from the Jurassic and Cretaceous periods. They appeared to be "stones that are perfect in form" and were set by European jewellers into magical rings and amulets from Medieval times up until the 18th century.