A toadstone

A toadstone, of circular cabochon form. 1.5cm long.

Toadstones, named for their resemblance to the bumps on the skin of a toad, are the fossilised teeth of Lepidotes, an extinct ray-finned fish of the Cretaceous and Jurrasic periods. Toads were thought to be poisonous, and so toadstones were worn set in jewellery from as early as the 14th century to protect the wearer from contagions and poisons.