The phrase can be used synonymously with "volcanic ash." It is actually mined from volcanic ash deposits, or it may be produced by crushing rock pumice. Rub the pumice stone in round motions more than the calloused space. Use Mild, circular motions to slough away the useless skin like corns https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/