The Pumice reference article from the English Wikipedia on 24-Apr-2004
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Pumice

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image:Different_rocks_at_Panum_Crater-300px.jpg
Top stone is obsidian, below that is pumice
and in lower right hand color is rhyolite (light color)
larger version
Pumice is a light, porous type of pyroclastic igneous rock. It is formed during explosive volcanic eruptions when liquid lava is ejected into the air as a froth containing masses of gas bubbles. As the lava solidifies, the bubbles are frozen into the rock. Any type of igneous rock - andesite, basalt, dacite or rhyolite - can form pumice given suitable eruptive conditions.

Image:Teidepumice.jpg
Specimen of highly porous pumice from Teide volcano
on Tenerife, Canary Islands. Density of specimen
approx 0.25 g/cm3. Scale in centimetres.

It is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water. It is widely used to make lightweight concrete and as an abrasive, especially in polishes and cosmetic exfoliants.

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