Pumice

Physical Properties

Hardness: 5-6 (but brittle/friable); Color: White, light gray, or cream; Luster: Pearly to dull; Crystal structure: Amorphous/Vitreous; Cleavage: None; Specific gravity: 0.25 to 0.7 (extremely low density)

Formation & Geological History

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid cooling and depressurization trap gas bubbles (vesicles) before the magma can crystallize, creating a volcanic glass foam.

Uses & Applications

Used as an abrasive in cosmetics (exfoliants) and cleaning supplies, in lightweight concrete and cinder blocks, for stone-washing denim jeans, and in horticulture as a soil amendment.

Geological Facts

Pumice is the only rock that can float on water due to its incredible porosity. Massive 'pumice rafts' thousands of miles long have been observed floating in the ocean after large underwater eruptions.

Field Identification & Locations

Identify by its light weight (low density), light color, and porous 'sponge-like' appearance. Commonly found in volcanic regions such as the Cascades, Italy, Greece, and New Zealand.

Identified on: 4/27/2026

Mode: Standard