pumice

pumice
    n 1: a light glass formed on the surface of some lavas; used as
         an abrasive [syn: {pumice}, {pumice stone}]
    v 1: rub with pumice, in order to clean or to smoothen
    

Pumice \Pum"ice\, n. [L. pumex, pumicis, prob. akin to spuma
   foam: cf. AS. pumic-st[=a]n. Cf. {Pounce} a powder, {Spume}.]
   (Min.)
   A very light porous volcanic scoria, usually of a gray color,
   the pores of which are capillary and parallel, giving it a
   fibrous structure. It is supposed to be produced by the
   disengagement of watery vapor without liquid or plastic lava.
   It is much used, esp. in the form of powder, for smoothing
   and polishing. Called also {pumice stone}.
   [1913 Webster]