The Collaborative International Dictionary of English v.0.48:

Toadstone \Toad"stone`\, n.
   1. (Min.) A local name for the igneous rocks of Derbyshire,
      England; -- said by some to be derived from the German
      todter stein, meaning dead stone, that is, stone which
      contains no ores.
      [1913 Webster]

   2. Bufonite, formerly regarded as a precious stone, and worn
      as a jewel. See Bufonite.
      [1913 Webster]