PUMICE
pumice, pumice stone
(noun) a light glass formed on the surface of some lavas; used as an abrasive
pumice
(verb) rub with pumice, in order to clean or to smoothen
Source: WordNet® 3.1
Etymology
Noun
pumice (countable and uncountable, plural pumices)
A light, porous type of pyroclastic igneous rock, formed during explosive volcanic eruptions when liquid lava is ejected into water or air as a froth containing masses of gas bubbles. As the lava solidifies, the bubbles are frozen into the rock.
Verb
pumice (third-person singular simple present pumices, present participle pumicing, simple past and past participle pumiced)
(transitive) To abrade or roughen with pumice.
Source: Wiktionary
Pum"ice, n. Etym: [L. pumex, pumicis, prob. akin to spuma foam: cf.
AS. pumic-stan. Cf. Pounce a powder, Spume.] (Min.)
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition