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