Top stone is obsidian, below that is pumice and in lower right hand color is
rhyolite (light color). 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.
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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