scoria

noun2

1. •
   The slag or dross that remains after the smelting of metal from an ore.

   • The like stuff is in Anacharsis: hot metal; full of scoriae, which should and could have been smelted out, but which will not. — 1837
2. •
   Rough masses of rock formed by solidified lava, and which can be found around a volcano's crater.

   • To the south lay broken shapes of scoria in a lava bed as far as the eye could see. — 1985