jew's-stone

from The Century Dictionary.

• noun The clavated fossil spine of a very large egg-shaped echinus.
• noun The basalt capping the coal-measures on the Titterstone and Brown Clee hills in Shropshire, England; also, the local name of a limestone-bed belonging to the White Lias (Rhætic) in Somersetshire.
• noun Crystallized iron pyrites or pyrite, early used as a gem.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Paleon.) A large clavate spine of a fossil sea urchin.