igneous
Pumice
Vesicular volcanic glass (primarily SiO2)
Hardness: 5-6 (but brittle), Color: grayish to dark gray/black (scoriaceous), Luster: dull to vitreous, Structure: vesicular (pitted with gas bubbles), Specific Gravity: 0.25 to 0.7 (often floats on water).
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Physical properties
Hardness: 5-6 (but brittle), Color: grayish to dark gray/black (scoriaceous), Luster: dull to vitreous, Structure: vesicular (pitted with gas bubbles), Specific Gravity: 0.25 to 0.7 (often floats on water).
Formation & geological history
Formed during explosive volcanic eruptions when pressurized lava is violently ejected. The rapid cooling and depressurization cause gas bubbles to be trapped in the solidifying rock, creating a frothy texture.
Uses & applications
Used as an abrasive in cleaning and personal care (pumice stones), in lightweight concrete production, as a landscaping material, and in stone-washing textiles like denim.
Geological facts
Pumice is the only rock that can float on water due to its low density and high porosity. Large 'pumice rafts' can form on the ocean after underwater volcanic eruptions and drift for thousands of miles.
Field identification & locations
Identify in the field by its extreme lightness, coarse vesicular texture, and ability to float. Commonly found in volcanic regions such as the Cascade Range (USA), Italy, and Iceland.
