toadstone

noun

1. (countable, uncountable)A small stone, once believed to be a jewel embedded in the head of a toad, worn as an amulet.
2. (countable, uncountable)A 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.”