scoria
noun
sco·ria ˈskȯr-ē-ə 
plural scoriae ˈskȯr-ē-ˌē
-ē-ˌī
1
: the refuse from melting of metals or reduction of ores : slag
2
: rough vesicular cindery lava
scoriaceous
ˌskȯr-ē-ˈā-shəs
adjective 
Example Sentences
Recent Examples on the Web As if the Easter Island statues weren’t enigmatic enough, a few of them are wearing hats—6.5-foot-wide, 13-ton cylinders of cindery red volcanic rock called scoria. 
Kiona N. Smith, Ars Technica, 5 June 2018
Kiona N. Smith, Ars Technica, 5 June 2018 Word History
Etymology
Middle English, from Latin, from Greek skōria, from skōr excrement — more at scatology
First Known Use
14th century, in the meaning defined at sense 1