igneous
Pumice
Pumice (Amorphous volcanic glass)
Hardness: 5-6 (but brittle), Color: Light gray to tan, Luster: Dull/pearly, Structure: Vesicular (highly porous/holey), Specific Gravity: 0.25-1.2 (can float in water)
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Physical properties
Hardness: 5-6 (but brittle), Color: Light gray to tan, Luster: Dull/pearly, Structure: Vesicular (highly porous/holey), Specific Gravity: 0.25-1.2 (can float in water)
Formation & geological history
Formed during explosive volcanic eruptions when gas-rich, frothy magma cools rapidly, trapping bubbles in volcanic glass. Typical of rhyolitic or dacitic eruptions.
Uses & applications
Used as an abrasive in skincare (exfoliation), heavy-duty cleaning, stone-washed denim production, lightweight concrete, and horticulture as a soil amendment.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large rafts of floating pumice can travel for thousands of miles across oceans.
Field identification & locations
Identify by its incredibly light weight, 'sponge-like' appearance with many tiny holes (vesicles), and rough texture. Commonly found in volcanic regions like the Cascades, Italy, or Iceland.
