igneous
Pumice
Pumice (Highly vesicular felsic extrusive igneous rock; primarily SiO2)
Hardness: 5-6 (Mohs), but brittle; Color: Light gray, white, or cream; Luster: Pearly to dull; Structure: Highly vesicular (foamy) with glass-walled bubbles; Specific Gravity: Below 1.0 (often floats in water).
- Hardness
- 5-6 (Mohs), but brittle
- Color
- Light gray, white, or cream
- Luster
- Pearly to dull
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Physical properties
Hardness: 5-6 (Mohs), but brittle; Color: Light gray, white, or cream; Luster: Pearly to dull; Structure: Highly vesicular (foamy) with glass-walled bubbles; Specific Gravity: Below 1.0 (often floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich molten rock (magma) is violently ejected. The rapid cooling and depressurization trap gas bubbles in a glass matrix. It is found in volcanic regions of all geological ages, especially near subduction zones.
Uses & applications
Used as a lightweight aggregate in concrete/cinder blocks, a decorative landscaping stone, an abrasive in 'stone-washed' jeans, and in personal care for exfoliating skin and polishing teeth.
Geological facts
Pumice is the only rock that can float on water due to its extreme porosity. After major volcanic eruptions, massive 'pumice rafts' can form on the ocean, some of which are large enough to be seen from space and can float for years.
Field identification & locations
Identify by its light weight and rough, scratchy texture; it feels almost like a solidified sponge. Common in locations like the Aegean Islands, the American West (Cascades), and Italy. Collectors look for pieces with unique colors or distinct fiber-like structures.
