toadstone

plural toadstones

1. countableuncountableA small stone, once believed to be a jewel embedded in the head of a toad, worn as an amulet.
2. countableuncountablegeographyA soft, earthy variety of trap-rock of a brownish-grey colour, looking like an argillaceous deposit.

   Rowley-Rag appeared to be the same thing as basalt and toadstone resembled some species of lava.

   2004, Robert E Schofield, The Enlightened Joseph Priestley, Pennsylvania State University, published 2004, page 162