The transpiring between previous volcanic rocks, the cavities are frequently full of deposits of secondary minerals introduced by percolating drinking water. The glass alone varieties threads, fibres, and slim partitions concerning the vesicles. Rhyolite and trachyte pumices are white, andesite pumices typically yellow or brown, and pumiceous basalts (for example https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/