igneous
Pumice
Pumiceous rhyolite / Volcanic glass
Mohs hardness: 5-6; Color: Light gray, cream, or off-white; Luster: Dull to pearly; Structure: Vesicular (highly porous/holey); Cleavage: None; Specific gravity: 0.25-0.90 (floats in water).
- Hardness
- 5-6
- Color
- Light gray, cream, or off-white
- Luster
- Dull to pearly
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Physical properties
Mohs hardness: 5-6; Color: Light gray, cream, or off-white; Luster: Dull to pearly; Structure: Vesicular (highly porous/holey); Cleavage: None; 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 ejected. The rapid cooling and depressurization trap gas bubbles as the glass solidifies instantly.
Uses & applications
Used as an abrasive in soaps (pumice stones) and toothpastes, in horticulture as a soil amendment, in lightweight concrete blocks, and for stone-washing denim jeans.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large eruptions can create 'pumice rafts' that drift for thousands of miles across the ocean.
Field identification & locations
Identify in the field by its extremely light weight and porous 'sponge-like' appearance. It is commonly found in areas of past or present volcanic activity, such as Italy, Turkey, and the Western USA.
