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]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "scoria":
   aa, abyssal rock, alluvion, alluvium, ash, ashes, basalt, bedrock,
   block lava, brand, brash, breccia, calx, carbon, charcoal, cinder,
   clinker, coal, coke, conglomerate, coom, crag, deposition,
   deposits, diluvium, draff, dregs, dross, druid stone, ember, feces,
   festooned pahoehoe, froth, fume, gneiss, granite, grounds,
   igneous rock, lava, lees, limestone, living rock, loess, magma,
   mantlerock, metamorphic rock, monolith, moraine, offscum, pahoehoe,
   pillow lava, porphyry, precipitate, precipitation, pudding stone,
   reek, regolith, rock, ropy lava, rubble, rubblestone, sandstone,
   sarsen, schist, scree, scum, sediment, sedimentary rock, settlings,
   shelly pahoehoe, silt, sinter, slag, smoke, smudge, smut, soot,
   stone, sublimate, sullage, talus, tufa, tuff