The developing between old volcanic rocks, the cavities are usually crammed with deposits of secondary minerals launched by percolating water. The glass itself varieties threads, fibres, and skinny 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/