igneous
Pumice
Pumice (volcanic glass foam)
Hardness: 5-6 (Mohs), Color: Light gray to tan, Luster: Dull to pearly, Crystal structure: Amorphous (glassy), Cleavage: None, Specific gravity: 0.25 to 0.7 (floats on water)
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Physical properties
Hardness: 5-6 (Mohs), Color: Light gray to tan, Luster: Dull to pearly, Crystal structure: Amorphous (glassy), Cleavage: None, Specific gravity: 0.25 to 0.7 (floats on water)
Formation & geological history
Formed during explosive volcanic eruptions when lava is saturated with gas and depressurized, creating a frothy, vesicular structure. It can be found in volcanic regions of all geological ages.
Uses & applications
Used as an abrasive in cleaning products, exfoliants in cosmetics, lightweight aggregate in concrete (pumicite), and for stone-washing denim.
Geological facts
Pumice is the only rock that can float on water due to its low density. Large eruptions can create 'pumice rafts' that drift across oceans for thousands of miles.
Field identification & locations
Identify by its extremely lightweight feel and vesicular (bubbly) texture. Common in the Cascades, Andes, and Mediterranean. To test, place it in a bowl of water to see if it floats.
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