1. pumice stone
Wikipedia definition
2. PumicePumice  /ˈpʌmɨs/ is a textural term for a volcanic rock which is a solidified frothy lava. Pumice is typically created when super-heated, highly pressurized rock is violently ejected from a volcano. Alternatively it can be formed when lava and water are mixed. Most pumice is light enough that it floats on water. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization.
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