igneous
Pumice
Pumice (a vesicular volcanic glass)
Hardness: 5-6 (but brittle); Color: White, cream, grey, or tan; Luster: Pearly to earthy; Structure: Highly vesicular (honeycomb-like pores); Specific Gravity: 0.25-0.90 (floats in water).
- Hardness
- 5-6 (but brittle)
- Color
- White, cream, grey, or tan
- Luster
- Pearly to earthy
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Physical properties
Hardness: 5-6 (but brittle); Color: White, cream, grey, or tan; Luster: Pearly to earthy; Structure: Highly vesicular (honeycomb-like pores); Specific Gravity: 0.25-0.90 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich molten rock (magma) is violently ejected and cools rapidly, trapping gas bubbles in a glass-like matrix.
Uses & applications
Used as an abrasive in soaps (pumice stones), cosmetic exfoliants, dental polishing, lightweight concrete aggregate, and horticultural soil drainage.
Geological facts
Pumice is the only rock that can float on water due to its low density and trapped gas pockets. Over time, it may waterlog and sink, but it can travel thousands of miles across oceans as 'pumice rafts'.
Field identification & locations
Identify by its extreme lightness (low density) and porous, sponge-like surface. Often found near active or dormant explosive volcanic regions like the Cascades or Mediterranean.
