igneous
Pumice
Pumice (extrusive volcanic rock)
Hardness: 5-6 (variable), Color: Light gray/tan/cream, Luster: Dull to silky, Crystal structure: Amorphous (volcanic glass), Cleavage: None, Specific gravity: 0.25 to 0.70 (floats on water)
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Physical properties
Hardness: 5-6 (variable), Color: Light gray/tan/cream, Luster: Dull to silky, Crystal structure: Amorphous (volcanic glass), Cleavage: None, Specific gravity: 0.25 to 0.70 (floats on water)
Formation & geological history
Formed during explosive volcanic eruptions when gas-rich lava is ejected and quickly cools, trapping gas bubbles (vesicles) in a glassy matrix. Common in subduction zones.
Uses & applications
Abrasives for personal care (pumice stones), horticultural soil amendments, lightweight concrete aggregate, pencil erasers, and stone-washed jeans.
Geological facts
Pumice is the only rock that floats on water. During massive eruptions, floating 'pumice rafts' can form in the ocean, covering thousands of square miles and lasting for years.
Field identification & locations
Identify by its light weight (low density) and highly porous, frothy texture. Found in volcanic regions like the Cascades, Andes, and Mediterranean. Excellent for beginner collectors.
