igneous
Pumice
Pumice (Amorphous volcanic glass)
Hardness: 5-6 (Mohs); Color: Light gray to cream; Luster: Dull to pearly; Structure: Vesicular (highly porous/frothy); Specific Gravity: 0.25-0.9 (floats in water).
- Hardness
- 5-6 (Mohs)
- Color
- Light gray to cream
- Luster
- Dull to pearly
Identified More igneous →
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Physical properties
Hardness: 5-6 (Mohs); Color: Light gray to cream; Luster: Dull to pearly; Structure: Vesicular (highly porous/frothy); Specific Gravity: 0.25-0.9 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected and cools rapidly, trapping air bubbles in the solidifying glass. Found in areas with geologically recent volcanic activity.
Uses & applications
Used as an abrasive in soaps, foot stones, and industrial polishing; lightweight aggregate in concrete; horticultural soil amendment; and in stone-washing textiles like blue jeans.
Geological facts
Pumice is the only rock that can reliably float on water because of its low density and air-trapped vesicles. Huge rafts of floating pumice can occasionally be seen in the ocean following submarine eruptions.
Field identification & locations
Identified by its extremely lightweight feel, light color, and porous, sponge-like texture. If it floats in water, it is almost certainly pumice or a related volcanic glass.
