Rhyolitic lavas are quite viscous, or thick, and contain large quantities of gas. The gas often boils off with explosive force, expelling large amounts of glowing ash and cinders. Sometimes, however, it is released more slowly or even trapped as bubbles when the lava hardens. When the bubbles are tiny and closely packed, a lightweight stone called pumice is formed. Any kind of lava may turn into pumice, but most of it develops in rhyolites.
Pumice is used commercially for cleaning and polishing wood, metal, and other surfaces.
More recently it has found use in precast masonry, poured concrete, insulation, acoustical tile, and plaster. Rhyolite pumices are white. Andesite pumices are usually yellow or brown. Basaltic pumices are black. Commercially useful pumices are found in the Canary Islands, Iceland, Italy, Hungary, Greece, New Zealand, and Germany.