From thunder + stone.

thunderstone (plural thunderstones)

1. (archaic) A thunderbolt.
2. A stone or fossil thought to be, or be caused by, a thunderbolt striking the earth.
   • 1985, Wade Davis, The Serpent and the Rainbow, Simon & Schuster, page 94:

     Thunderstones are sacred to the vodounist, forged as they are by Sobo and Shango, the spirits of thunder and lightning.

3. A belemnite.