The happening amid previous volcanic rocks, the cavities are frequently stuffed with deposits of secondary minerals launched by percolating drinking water. The glass alone kinds threads, fibres, and slender partitions among the vesicles. Rhyolite and trachyte pumices are white, andesite pumices frequently 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/