The transpiring between old volcanic rocks, the cavities are usually stuffed with deposits of secondary minerals launched by percolating drinking water. The glass itself sorts threads, fibres, and thin partitions in between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices frequently 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/