igneous
Pumice
Pumice (Amorphous volcanic glass)
Hardness: 5-6 (but very friable); Color: White, cream, or light grey; Luster: Dull to pearly; Structure: Vesicular (highly porous); Specific Gravity: 0.25 to 0.9 (floats in water).
- Hardness
- 5-6 (but very friable)
- Color
- White, cream, or light grey
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (but very friable); Color: White, cream, or light grey; Luster: Dull to pearly; Structure: Vesicular (highly porous); Specific Gravity: 0.25 to 0.9 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when high-pressure, gas-charged molten rock is ejected and cools rapidly, trapping gas bubbles in a frothy glass structure.
Uses & applications
Used as an abrasive in toothpaste and cosmetics (exfoliants), in construction for lightweight concrete, in horticulture as a soil amendment, and for stone-washing denim.
Geological facts
Pumice is the only rock that consistently floats on water due to its incredible number of vacuum-sealed air cells. Huge 'pumice rafts' can form in the ocean after underwater volcanic eruptions.
Field identification & locations
Identify in the field by its extreme lightness (low density) and distinct sponge-like, bubbly appearance. Look for it near felsic volcanic vents or washed up on coastlines.
