"stone or stone-like object, supposedly magical (with healing or protective power) and found in the heads of certain toads," 1550s, from toad + stone (n.). Translating Greek batrakhites, Medieval Latin bufonites; compare also French crapaudine (13c.), German krötenstein.
The Crapadine or Toadstone, we speak of before, you shall prove to be a true one, if the Toad lift himself so up against it, when it is shew'd or held to him, as if he would come at it, and leap to catch it away: he doth so much envy that Man should have that stone. ["Eighteen Books of the Secrets of Art & Nature," 1661]