igneous
Pumice
Pumice (Vesicular Rhyolite/Andesite)
Hardness: 5-6 Mohs (though brittle); Color: White, cream, or light grey; Luster: Dull to pearly; Crystal structure: Amorphous/Glassy; Cleavage: None; Specific gravity: 0.25–0.9 (floats in water).
- Hardness
- 5-6 Mohs (though brittle)
- Color
- White, cream, or light grey
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 Mohs (though brittle); Color: White, cream, or light grey; Luster: Dull to pearly; Crystal structure: Amorphous/Glassy; Cleavage: None; Specific gravity: 0.25–0.9 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when pressurized lava is violently ejected from a volcano. The rapid depressurization and cooling trap gas bubbles (vesicles) within the cooling glass, creating a frothy texture. Most specimens are relatively young geologically (Cenozoic).
Uses & applications
Used as a lightweight aggregate in concrete, as an abrasive in 'pumice stones' for skincare, in stone-washing textiles (jeans), and as a filtration medium or soil amendment in horticulture.
Geological facts
Pumice is the only rock that can float on water until it becomes waterlogged. Massive 'pumice rafts' can form in the ocean after underwater volcanic eruptions, sometimes drifting for thousands of miles and providing transport for marine organisms.
Field identification & locations
Identify in the field by its extremely light weight, frothy 'sponge-like' appearance, and ability to float. It is commonly found in volcanic regions like the Cascades (USA), Italy, Greece, and Iceland.
