The occurring between aged volcanic rocks, the cavities are frequently filled with deposits of secondary minerals released by percolating h2o. The glass by itself forms threads, fibres, and skinny partitions in between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices generally yellow or brown, and pumiceous basalts (for https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/