Igneous
Pumice
Pumice (a highly vesicular extrusive igneous rock)
Hardness: 5-6 Mohs; Color: Light gray, tan, or cream; Luster: Pearly to dull; Structure: Vesicular (gas-filled bubbles), amorphous (glassy); Specific Gravity: 0.25 - 0.7 (often floats in water).
- Hardness
- 5-6 Mohs
- Color
- Light gray, tan, or cream
- Luster
- Pearly to dull
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Physical properties
Hardness: 5-6 Mohs; Color: Light gray, tan, or cream; Luster: Pearly to dull; Structure: Vesicular (gas-filled bubbles), amorphous (glassy); Specific Gravity: 0.25 - 0.7 (often floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich rhyolitic or andesitic lava is violently ejected. The rapid cooling and depressurization trap gas bubbles, creating a porous glass foam.
Uses & applications
Used as an abrasive in polishing and cleaning, as a lightweight aggregate in concrete/cinder blocks (horticulture), and in personal care as a pumice stone for exfoliation.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged. Large eruptions can create 'pumice rafts' in the ocean that are hundreds of miles long.
Field identification & locations
Identify by its light weight (low density) and the presence of numerous tiny pits or vesicles. Commonly found in volcanic regions, along shorelines after eruptions, or in decorative landscaping gravel.
