igneous
Pumice
Pumice (a highly vesicular volcanic glass)
Hardness: 5-6 (but brittle), Color: White, cream, or light gray, Luster: Dull to pearly, Structure: Vesicular (full of air bubbles), Specific Gravity: 0.25-0.90 (floats in water)
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Physical properties
Hardness: 5-6 (but brittle), Color: White, cream, or light gray, Luster: Dull to pearly, Structure: Vesicular (full of air bubbles), Specific Gravity: 0.25-0.90 (floats in water)
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich molten rock is ejected and cools rapidly, trapping air bubbles in a glass-like structure.
Uses & applications
Used as an abrasive in soaps (Lava soap), exfoliating stones, dental prophylaxis, stonewashing jeans, and as a lightweight aggregate in concrete and plaster.
Geological facts
Pumice is the only rock that floats on water until it eventually becomes waterlogged and sinks. Large eruptions can create 'pumice rafts' that drift for thousands of miles across oceans.
Field identification & locations
Identify by its extremely lightweight feel and sponge-like appearance with many tiny holes (vesicles). It is commonly found near volcanic arcs and subduction zones.
