igneous
Pumice
Pumice (volcanic glass froth)
Hardness: 5-6 Mohs (though brittle). Color: light gray, white, or cream. Luster: dull to pearly. Structure: vesicular (pitted with air bubbles). Specific gravity: very low (often floats on water).
- Hardness
- 5-6 Mohs (though brittle)
- Color
- light gray, white, or cream
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 Mohs (though brittle). Color: light gray, white, or cream. Luster: dull to pearly. Structure: vesicular (pitted with air bubbles). Specific gravity: very low (often floats on water).
Formation & geological history
Formed during violent explosive volcanic eruptions when highly pressurized, gas-rich molten rock (magma) is ejected and cools rapidly, trapping gas bubbles in a frothy glass matrix.
Uses & applications
Used as an abrasive in soaps (Lava soap), exfoliating stones for skincare, lightweight aggregate in concrete/cinder blocks, and in stonewashed denim production.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large eruptions can create 'pumice rafts' that drift for miles across the ocean.
Field identification & locations
Identify by its extremely lightweight feel and sponge-like texture. Commonly found in volcanic regions like the Cascades (USA), Italy, and Iceland. Testing for buoyancy is a primary identification method.
