The transpiring between old volcanic rocks, the cavities usually are full of deposits of secondary minerals released by percolating water. The glass by itself kinds threads, fibres, and skinny partitions among the vesicles. Rhyolite and trachyte pumices are white, andesite pumices often yellow or brown, and pumiceous basalts (for instance https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/