The happening amongst aged volcanic rocks, the cavities tend to be filled with deposits of secondary minerals introduced by percolating drinking water. The glass itself kinds threads, fibres, and skinny partitions among the vesicles. Rhyolite and trachyte pumices are white, andesite pumices normally 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/