igneous
Pumice
Pumice (Vesicular volcanic glass)
Hardness: 5-6 (Mohs), Color: Cream, white, or light gray, Luster: Dull/pearly, Structure: Highly vesicular/frisky glass, Cleavage: None, Specific Gravity: 0.25 to 0.7 (floats on water)
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Physical properties
Hardness: 5-6 (Mohs), Color: Cream, white, or light gray, Luster: Dull/pearly, Structure: Highly vesicular/frisky glass, Cleavage: None, Specific Gravity: 0.25 to 0.7 (floats on water)
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is violently ejected and cools rapidly, trapping gas bubbles as vesicles. Found in subduction zones worldwide.
Uses & applications
Used as an abrasive in soaps and cleaners, in horticultural soil mixes, for stonewashing denim, and as a lightweight aggregate in concrete.
Geological facts
Pumice is the only rock that can float on water due to its air-filled pores. It has been used since ancient times for hygiene; the Romans used it in construction to make lightweight dome structures like the Pantheon.
Field identification & locations
Identify in the field by its sponge-like appearance and extremely light weight. If it floats in a puddle, it is pumice. Look for it near active or dormant volcanic regions and beaches where it may have washed ashore.
