The Collaborative International Dictionary of English v.0.48:

Scoria \Sco"ri*a\, n.; pl. Scoriae. [L., fr. Gr. ?, fr. ?
   dung, ordure.]
   1. The recrement of metals in fusion, or the slag rejected
      after the reduction of metallic ores; dross.
      [1913 Webster]

   2. Cellular slaggy lava; volcanic cinders.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scoria
    n 1: the scum formed by oxidation at the surface of molten
         metals [syn: slag, scoria, dross]