The developing amongst previous volcanic rocks, the cavities tend to be filled with deposits of secondary minerals launched by percolating drinking water. The glass by itself forms threads, fibres, and thin partitions between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices generally yellow or brown, and pumiceous basalts https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/