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From Biology-Online Dictionary | Biology-Online Dictionary
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(Science: chemical) A very light porous volcanic scoria, usually of a gray colour, 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, especially. In the form of powder, for smoothing and polishing.

Synonym: pumice stone.

Origin: L. Pumex, pumicis, prob. Akin to spuma foam: cf. AS. Pumic-stan. Cf. Pounce a powder, Spume.