igneous
Pumice
Pumice (a highly vesicular extrusive volcanic glass)
Hardness: 5-6 (but brittle), Color: Light gray to white/cream, Luster: Dull or silky, Crystal structure: Amorphous/glassy, Specific gravity: Typically less than 1.0 (it floats)
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Physical properties
Hardness: 5-6 (but brittle), Color: Light gray to white/cream, Luster: Dull or silky, Crystal structure: Amorphous/glassy, Specific gravity: Typically less than 1.0 (it floats)
Formation & geological history
Formed during explosive volcanic eruptions where gas-rich magma cools very rapidly, trapping bubbles of steam and volcanic gas within the solidifying glass.
Uses & applications
Used as an abrasive in cosmetics (pumice stones), horticultural soil additives, lightweight concrete aggregates, and in stone-washing jeans.
Geological facts
Pumice is the only rock that can reliably float on water for long periods. After the Krakatoa eruption in 1883, rafts of pumice floated in the ocean for years.
Field identification & locations
Identify by its extremely lightweight feel, porous (bubbly) texture, and ability to float on water. Commonly found in volcanic regions or washed up on beaches.
