igneous
Pumice
Pumice (Vesicular Rhyolite/Andesite)
Hardness: 5-6 (can be brittle); Color: Cream, light gray, or off-white; Luster: Dull to pearly; Crystal structure: Amorphous/Glassy; Specific Gravity: < 1.0 (floats in water); Texture: Highly vesicular (honeycombed with air bubbles).
- Hardness
- 5-6 (can be brittle)
- Color
- Cream, light gray, or off-white
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (can be brittle); Color: Cream, light gray, or off-white; Luster: Dull to pearly; Crystal structure: Amorphous/Glassy; Specific Gravity: < 1.0 (floats in water); Texture: Highly vesicular (honeycombed with air bubbles).
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 in the solidifying glass. Found in volcanic arcs and continental volcanic regions.
Uses & applications
Used as a lightweight aggregate in concrete, as an abrasive in polishing compounds and cosmetics (exfoliants), in horticulture to improve soil aeration, and for stone-washing denim.
Geological facts
Pumice is the only rock that can float on water due to its incredible porosity. After large undersea eruptions, 'pumice rafts' can form and float across oceans for years, sometimes carrying entire ecosystems of barnacles and crabs.
Field identification & locations
Identify by its extremely light weight and porous 'sponge-like' appearance. If it floats in a glass of water, it is almost certainly pumice. Commonly found near active or dormant volcanic sites.
