igneous
Pumice
Vesicular Rhyolite or Dacite Pumice (Amorphous volcanic glass)
Hardness: 5-6 Mohs (though brittle and easy to crush); Color: light gray, tan, or cream; Luster: dull to pearly; Structure: highly vesicular (pitted with gas bubbles); Specific Gravity: 0.25 to 0.70 (will often float in water).
- Hardness
- 5-6 Mohs (though brittle and easy to crush)
- Color
- light gray, tan, or cream
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 Mohs (though brittle and easy to crush); Color: light gray, tan, or cream; Luster: dull to pearly; Structure: highly vesicular (pitted with gas bubbles); Specific Gravity: 0.25 to 0.70 (will often float in water).
Formation & geological history
Formed during explosive volcanic eruptions when magma with high water and gas content is depressurized. The rapid cooling traps gas bubbles, creating a solidified froth. Commonly found in young volcanic belts (Cenozoic to modern age).
Uses & applications
Used as an abrasive in cosmetics (exfoliants), household cleaners, and stonewashed jeans. In construction, it is used as a lightweight aggregate for concrete and as horticultural soil amendment.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes water-logged. Large eruptions can create 'pumice rafts' in the ocean that are hundreds of miles long and can drift for years.
Field identification & locations
Identify by its extremely lightweight feel, abrasive texture, and presence of tiny voids. Found in volcanic regions like the Cascades, Andes, or Mediterranean. Collect from dry wash beds or volcanic ash deposits.
