The transpiring among the old volcanic rocks, the cavities are usually full of deposits of secondary minerals launched by percolating h2o. The glass alone types threads, fibres, and slim partitions between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices generally yellow or brown, and pumiceous basalts (for instance https://bookmarkingfeed.com/story19339749/pumice-stone-wholesalers-fundamentals-explained