igneous
Pumice
Pumiceous rhyolite
Hardness: 5-6 Mohs (is fragile and can crumble); Color: white, cream, tan, or grey; Luster: vitreous to dull; Structure: vesicular (pitted with gas bubbles); Specific Gravity: 0.25-0.7 (floats on water).
- Hardness
- 5-6 Mohs (is fragile and can crumble)
- Color
- white, cream, tan, or grey
- Luster
- vitreous to dull
Identified More igneous →
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Physical properties
Hardness: 5-6 Mohs (is fragile and can crumble); Color: white, cream, tan, or grey; Luster: vitreous to dull; Structure: vesicular (pitted with gas bubbles); Specific Gravity: 0.25-0.7 (floats on water).
Formation & geological history
Formed during explosive volcanic eruptions when pressurized, gas-rich molten rock (magma) is ejected into the air; it cools rapidly, freezing gas bubbles inside to create a frothy glass.
Uses & applications
Used as an abrasive in polishing, skin exfoliation (pumice stones), heavyweight-reducing concrete aggregate, and stone-washing blue jeans.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large volcanic eruptions can create 'pumice rafts' in the ocean that travel for miles.
Field identification & locations
Identify by its extremely light weight and porous 'sponge-like' texture. Commonly found in volcanic regions such as the Cascades, Italy, or Iceland. If it floats, it is almost certainly pumice.
