pumice

n. (in full pumice-stone) 1 light porous volcanic rock used in cleaning or polishing. 2 piece of this used for removing hard skin etc. [latin pumex pumic-]

n. 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.