Toadstone

Toadstones were actually the button-like fossilised teeth of Scheenstia (previously 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 until the 18th century.

[1] The true toadstone was taken by contemporary jewellers to be no bigger than the nail of a hand and they varied in colour from a whitish brown through green to black, depending on where they were buried.

[4] Alternatively Johannes de Cuba, in his book Gart der Gesundheit of 1485, claimed that toadstone would help with kidney disease and earthly happiness.

The toadstone is alluded to by Duke Senior in Shakespeare's As You Like It (1599), in Act 2, Scene 1, lines 12 to 14: Sweet are the uses of adversity; Which, like the toad, ugly and venomous, Wears yet a precious jewel in his head.