Igneous
Pumice
Pumicite (Amorphous glass with varying chemical composition, often rhyolitic)
Hardness: 5-6 (Mohs); Color: Pale grey, tan, or yellowish-white; Luster: Pearly to dull; Structure: Vesicular (porous/holohyaline); Specific Gravity: 0.25-0.90 (often floats on water); Cleavage: None.
- Hardness
- 5-6 (Mohs)
- Color
- Pale grey, tan, or yellowish-white
- Luster
- Pearly to dull
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Physical properties
Hardness: 5-6 (Mohs); Color: Pale grey, tan, or yellowish-white; Luster: Pearly to dull; Structure: Vesicular (porous/holohyaline); Specific Gravity: 0.25-0.90 (often floats on water); Cleavage: None.
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected into the atmosphere. The rapid cooling and depressurization trap gas bubbles (vesicles) within the rock as it solidifies. Primarily associated with rhyolitic, dacitic, and andesitic volcanism throughout geological history.
Uses & applications
Used as an abrasive in soaps (Lava soap), dental polishing pastes, and skin exfoliants (pumice stones). In construction, it is used as a lightweight aggregate for concrete and cinder blocks. It is also used in horticulture for soil aeration and as a filtration medium.
Geological facts
Pumice is the only rock that can float on water due to its low density and high porosity. Large 'pumice rafts' can form on the ocean after underwater volcanic eruptions and can float for years, sometimes carrying biological organisms across entire oceans.
Field identification & locations
Identify in the field by its extremely light weight relative to its size and its porous, sponge-like texture. It is commonly found in areas of recent volcanic activity such as the Andes, Italy, Greece, and the Western United States. Collectors should look for unique vesicular patterns or inclusions of obsidian.
