The taking place amongst previous volcanic rocks, the cavities are generally full of deposits of secondary minerals launched by percolating water. The glass alone forms threads, fibres, and slender partitions between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices normally 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/