igneous
Pumice
Pumice (Vesicular Rhyolite/Andesite)
Hardness: 5-6 (Mohs), Color: Light gray to white/cream, Luster: Dull to silky, Crystal structure: Amorphous/Vitreous, Cleavage: None, Specific gravity: 0.2-0.9 (floats in water)
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Physical properties
Hardness: 5-6 (Mohs), Color: Light gray to white/cream, Luster: Dull to silky, Crystal structure: Amorphous/Vitreous, Cleavage: None, Specific gravity: 0.2-0.9 (floats in water)
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid cooling and depressurization create a frothy, vesicular glass foam.
Uses & applications
Used as a lightweight aggregate in concrete, as an abrasive in soaps and polishes, for stone-washing textiles (jeans), and as a horticultural soil amendment.
Geological facts
Pumice is the only rock that can reliably float on water for long periods due to its high porosity. Entire floating 'pumice rafts' can form in the ocean after massive eruptions.
Field identification & locations
Identify by its extremely lightweight feel and vesicular (bubbly) texture. It is commonly found in volcanic regions such as the Ring of Fire or Iceland.
