Pumice
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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