scoria

Etymology

Borrowed from Latin scōria, from Ancient Greek σκωρία (skōría), from σκῶρ (skôr, “dung”).

noun

1. The slag or dross that remains after the smelting of metal from an ore.
2. (geology) Rough masses of rock formed by solidified lava, and which can be found around a volcano's crater.
   An excellent guidebook by Drs Kilburn and McGuire of University College London reveals that these unpromising pieces of debris are scoria and lithic fragments of the March 1944 eruption. 2004, Richard Fortey, The Earth, Folio Society, published 2011, page 10