The occurring among the aged volcanic rocks, the cavities are usually stuffed with deposits of secondary minerals launched by percolating water. The glass alone kinds threads, fibres, and thin partitions concerning the vesicles. Rhyolite and trachyte pumices are white, andesite pumices generally yellow or brown, and pumiceous basalts (for instance https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/