Pumice stone is formed when molten volcanic glass, ejected from beneath the earth’s crust, cools so rapidly that there is no time for it to crystallize.
After the pumice has solidified, the gases inside are suddenly released and the stone swells up into its characteristics light and airy form. If the substance had cooled under greater pressure it would have turned into solid glass.
The stones have long been used for cleaning and polishing. Since the Second World War it has been employed widely in railroad building, masonry and insulation. Good pumice is found in Iceland, the Canaries, New Zealand, Greece, the pacific coast of the United States and many other areas with a volcanic background.