The happening among aged volcanic rocks, the cavities usually are crammed with deposits of secondary minerals launched by percolating water. The glass alone forms threads, fibres, and slim partitions between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices generally yellow or brown, and pumiceous basalts (for example manifest https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/