The developing between aged volcanic rocks, the cavities are often full of deposits of secondary minerals launched by percolating water. The glass itself varieties threads, fibres, and thin partitions among the vesicles. Rhyolite and trachyte pumices are white, andesite pumices typically yellow or brown, and pumiceous basalts (including happen from https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/