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Perforated by the explosion of steam as it escapes from a volcano and puffed up into a froth of glass, pumice is a foamy mass of silky glass fibers. Pumice's porous nature and many isolated cells of air allow it to float on water for a long time. Pumice's composition ranges from acidic (high silica) to basic (low silica), but is usually regarded as acidic. Most pumice is light-colored, but some may be red or brown.
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