igneous
Pumice
Vesicular Rhyolite or Andesite (Amorphous volcanic glass)
Hardness: 5-6 (but very brittle); Color: Light gray, cream, or yellowish; Luster: Dull to pearly; Crystal structure: Amorphous (glassy); Specific gravity: 0.25–0.9 (floats in water); highly vesicular texture with numerous small cavities.
- Hardness
- 5-6 (but very brittle)
- Color
- Light gray, cream, or yellowish
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (but very brittle); Color: Light gray, cream, or yellowish; Luster: Dull to pearly; Crystal structure: Amorphous (glassy); Specific gravity: 0.25–0.9 (floats in water); highly vesicular texture with numerous small cavities.
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich lava is ejected into the air. The rapid cooling and depressurization trap gas bubbles as the lava solidifies into glass. They are found in volcanic regions worldwide and can vary in age from modern eruptions to millions of years old.
Uses & applications
Used as a lightweight aggregate in concrete (pumicite), as an abrasive in soaps (Lava soap), for stone-washing denim, in horticulture to aerate soil, and as an exfoliant in beauty products (pumice stones).
Geological facts
Pumice is the only rock that can float on water due to its incredible porosity. After massive volcanic eruptions, like Krakatoa or Tonga, 'pumice rafts' can form in the ocean, spanning hundreds of miles and floating for years.
Field identification & locations
Identify in the field by its extremely light weight relative to size and its ability to float in water. It feels rough and sandpaper-like. Commonly found in volcanic belts like the Ring of Fire (USA, Japan, Iceland, Italy).
