igneous
Pumice
Pumice (a highly vesicular volcanic glass)
Hardness: 5-6 Mohs (though fragile due to pores); Color: White, cream, or light grey; Luster: Dull to pearly; Structure: Vesicular (full of holes/bubbles); Specific Gravity: Sub-1.0 (floats in water).
- Hardness
- 5-6 Mohs (though fragile due to pores)
- Color
- White, cream, or light grey
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 Mohs (though fragile due to pores); Color: White, cream, or light grey; Luster: Dull to pearly; Structure: Vesicular (full of holes/bubbles); Specific Gravity: Sub-1.0 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when lava is ejected into the air; rapid cooling and gas pressure create a frothy, glass-like structure. Commonly found in subduction zone volcanic regions like the Andes or Cascade Range.
Uses & applications
Used as an abrasive in cleaning supplies, polishes, and exfoliants (pumice stones). In construction, it is used as a lightweight aggregate for concrete and as a soil amendment in horticulture.
Geological facts
Pumice is the only rock that can reliably float on water for long periods of time. In 1883, after the eruption of Krakatoa, pumice rafts floated across the Indian Ocean for up to two years.
Field identification & locations
Identify by its light weight and 'sponge-like' appearance. It is found near active or extinct volcanoes; collectors should look for specimens that feel unexpectedly light for their size. If it floats in a bucket of water, it's pumice.
