Pumice

Physical Properties

Mohs hardness: 5-6; Color: Light gray, cream, or tan; Luster: Dull to pearly; Crystal structure: Amorphous/Vitreous; 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 silica-rich magma is ejected. The rapid depressurization and cooling trap gas bubbles (vesicles) in the solidifying volcanic glass.

Uses & Applications

Used as an abrasive in soaps (Lava soap), foot scrubs, pencil erasers, and stone-washed jeans. Also used in lightweight concrete, horticulture (soil aeration), and filtration.

Geological Facts

Pumice is the only rock that can float on water until it becomes waterlogged. Massive 'pumice rafts' can form in the ocean after large underwater volcanic eruptions, traveling thousands of miles.

Field Identification & Locations

Identified by its extremely lightweight, frothy texture and numerous visible pores (vesicles). It is commonly found near volcanic arcs and subduction zones where rhyolitic or dacitic lavas occur.

Identified on: 5/5/2026

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