scoria vs. pumice

noun

plural

scoriae 
  1. Metallurgy.,  the refuse, dross, or slag left after melting or smelting metal; scum.

  2. Geology.,  a cinderlike basic cellular lava.

noun

  1. Also called pumice stonea porous or spongy form of volcanic glass, used as an abrasive.

verb (used with object)

pumiced, pumicing 
  1. to rub, smooth, clean, etc., with pumice.