igneous
Pumice
Vesicular volcanic glass (Amorphous silica mixed with various oxides)
Hardness: 5-6 Mohs; Color: Light gray, tan, or cream; Luster: Vitreous to dull; Structure: Highly vesicular/porous; Crystal Structure: Amorphous (glassy); Specific Gravity: 0.25-0.9 (floats in water).
- Hardness
- 5-6 Mohs
- Color
- Light gray, tan, or cream
- Luster
- Vitreous to dull
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Physical properties
Hardness: 5-6 Mohs; Color: Light gray, tan, or cream; Luster: Vitreous to dull; Structure: Highly vesicular/porous; Crystal Structure: Amorphous (glassy); 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 into the atmosphere. Rapid cooling and depressurization create bubbles (vesicles) trapped in solidifying glass.
Uses & applications
Used as an abrasive in soaps, toothpastes, and heavy-duty cleaners. Extensively used in horticulture as a soil amendment, in construction for lightweight concrete, and in garment manufacturing for stonewashing denim.
Geological facts
Pumice is the only rock that can float on water due to its incredible porosity. Massive 'pumice rafts' have been known to float across oceans for years following major volcanic eruptions.
Field identification & locations
Identify in the field by its sponge-like appearance and extremely light weight. If it floats in a puddle or stream, it is likely pumice or scoria (though scoria usually sinks). Found in volcanic regions like the Cascades (USA), Italy, and Iceland.
