The occurring among old volcanic rocks, the cavities are generally stuffed with deposits of secondary minerals introduced by percolating drinking water. The glass alone kinds threads, fibres, and slim partitions amongst the vesicles. Rhyolite and trachyte pumices are white, andesite pumices frequently yellow or brown, and pumiceous basalts (for instance https://ledbookmark.com/story5084892/a-review-of-pumice-stone