The developing among outdated volcanic rocks, the cavities tend to be stuffed with deposits of secondary minerals introduced by percolating water. The glass alone varieties threads, fibres, and skinny partitions among the vesicles. Rhyolite and trachyte pumices are white, andesite pumices frequently 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/