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