igneous
Pumice
Pumice (volcanic glass with vesicular texture)
Hardness: 5-6 Mohs scale; Color: typically white, cream, or light grey; Luster: dull to pearly; Crystal structure: amorphous (glassy); Specific gravity: 0.25–1.2 (low enough to float in water); Cleavage: none.
- Hardness
- 5-6 Mohs scale
- Color
- typically white, cream, or light grey
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 Mohs scale; Color: typically white, cream, or light grey; Luster: dull to pearly; Crystal structure: amorphous (glassy); Specific gravity: 0.25–1.2 (low enough to float in water); Cleavage: none.
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich silicate lava cools rapidly. The depressurization creates bubbles (vesicles) as the lava solidifies into rock. Found in regions of active or recently extinct volcanism.
Uses & applications
Used as an abrasive in cosmetics (pumice stones), toothpastes, and heavy-duty hand soaps. In construction, it is used as a lightweight aggregate for concrete and as a landscaping material. It is also used in the garment industry for 'stone-washing' denim.
Geological facts
Pumice is the only rock that can float on water due to its air-filled vesicles. Large eruptions can create 'pumice rafts' that drift for thousands of miles across oceans. It is a volcanic glass, meaning its cooling was too rapid for crystals to grow.
Field identification & locations
Identify in the field by its extreme lightweight feel relative to its size and its porous, sponge-like texture. Feel for a rough, abrasive surface. Most common in mountain ranges near tectonic plate boundaries like the Andes or Cascades.
