igneous
Pumice
Vesicular volcanic glass (primarily Alumino-silicates)
Hardness: 5-6 (but brittle), Color: White, cream, grey, or light tan, Luster: Dull to pearly, Structure: Highly vesicular (honeycombed with air bubbles), Specific Gravity: Very low (0.25–1.2), often floats on water.
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Physical properties
Hardness: 5-6 (but brittle), Color: White, cream, grey, or light tan, Luster: Dull to pearly, Structure: Highly vesicular (honeycombed with air bubbles), Specific Gravity: Very low (0.25–1.2), often floats on water.
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich silicate lava is violently ejected. The rapid cooling and depressurization lock air bubbles into the solidifying glass matrix.
Uses & applications
Used as an abrasive in skincare (exfoliation), heavy-duty cleaning products, polishing compounds for glass and metal, and as a lightweight aggregate in concrete and gardening (soil aeration).
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged. In a large eruption, 'pumice rafts' can form on the ocean and drift for thousands of miles.
Field identification & locations
Identify in the field by its extreme lightness (low density) and frothy, sponge-like texture. It is commonly found in areas of recent or past explosive volcanic activity, such as Italy, Greece, the American West, and Iceland.
