The happening amongst old volcanic rocks, the cavities are usually stuffed with deposits of secondary minerals introduced by percolating h2o. The glass itself types threads, fibres, and slender partitions in between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices often yellow or brown, and pumiceous basalts (such as https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/