igneous
Pumice
Pumice (a highly vesicular volcanic glass)
Hardness: 5-6 (but very brittle); Color: Light gray, white, or cream; Luster: Dull to pearly; Crystal structure: Amorphous/Vitreous; Cleavage: None; Specific Gravity: Below 1.0 (floats on water); Texture: Vesicular (full of tiny holes).
- Hardness
- 5-6 (but very brittle)
- Color
- Light gray, white, or cream
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (but very brittle); Color: Light gray, white, or cream; Luster: Dull to pearly; Crystal structure: Amorphous/Vitreous; Cleavage: None; Specific Gravity: Below 1.0 (floats on water); Texture: Vesicular (full of tiny holes).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected and cools rapidly. The decompression creates bubbles (vesicles) trapped in the solidifying glass. Age can vary from modern Holocene to millions of years old.
Uses & applications
Used as an abrasive in cleaning (pumice stones), in skincare (exfoliation), in lightweight concrete, as a filtration medium, and in 'stone-washing' denim for apparel.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large 'pumice rafts' can form on the ocean after underwater volcanic eruptions and drift for thousands of miles.
Field identification & locations
Identify by its extreme lightness (low density), light color, and 'sponge-like' texture with many visible holes. It won't scratch easily with a fingernail but will float. Commonly found near active or dormant explosive volcanoes (e.g., Cascades, Mediterranean).
