igneous
Pumice
Vesicular Rhyolite or Dacite
Hardness: 5-6 (Mohs scale), but often crumbles; Color: White, cream, gray, or light brown; Luster: Dull to pearly; Crystal Structure: Amorphous/Glassy; Cleavage: None; Specific Gravity: < 1.0 (floats on water); Texture: Highly vesicular (honeycombed with gas bubbles).
- Hardness
- 5-6 (Mohs scale), but often crumbles
- Color
- White, cream, gray, or light brown
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (Mohs scale), but often crumbles; Color: White, cream, gray, or light brown; Luster: Dull to pearly; Crystal Structure: Amorphous/Glassy; Cleavage: None; Specific Gravity: < 1.0 (floats on water); Texture: Highly vesicular (honeycombed with gas bubbles).
Formation & geological history
Formed during explosive volcanic eruptions where highly pressurized, gas-rich molten rock (magma) is violently ejected. The rapid cooling and depressurization trap gas bubbles inside the cooling glass. Geological age can range from very recent to hundreds of millions of years in preserved volcanic arcs.
Uses & applications
Used as an abrasive in industrial cleaners, toothpastes, and 'stone-washed' jeans; as a lightweight aggregate in concrete/cinder blocks; in horticulture (soil aeration); and as a cosmetic exfoliant (pumice stones).
Geological facts
Pumice is the only rock that can float on water due to its extreme porosity. Following the 1883 eruption of Krakatoa, rafts of pumice drifted across the Indian Ocean for up to 20 years, some large enough to support growing trees.
Field identification & locations
Identify in the field by its very light weight relative to size and its scratchy, 'foamy' surface texture. Commonly found in volcanic regions like the Cascades (USA), Italy, and New Zealand. Collectors look for light-colored, highly porous specimens without heavy weathering.
