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. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite.

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.

When used as an additive for cement, fine-grained version of pumice called pozzolan is mixed with lime to form a light-weight, smooth, plaster-like concrete. This form of concrete was used as far back as Roman times.