The transpiring amid outdated volcanic rocks, the cavities are usually stuffed with deposits of secondary minerals launched by percolating drinking water. The glass itself types threads, fibres, and slender partitions involving 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/