Pumice

Physical Properties

Hardness: 6 (Mohs scale); Color: light grey, white, or cream; Luster: dull to pearly; Crystal structure: Amorphous (glassy); Specific gravity: 0.25 to 0.7 (floats on water); Texture: highly vesicular (full of holes).

Formation & Geological History

Formed during explosive volcanic eruptions when highly pressurized, gas-rich lava is ejected into the air. The rapid cooling and depressurization freeze bubbles in place, creating a frothy rock. Predominantly from the Cenozoic era to modern eruptions.

Uses & Applications

Used as an abrasive in cosmetics (pumice stones), stone-washing denim, industrial polishing, lightweight concrete aggregates, and horticulture as a soil amendment.

Geological Facts

Pumice is the only rock that can float on water due to its air-filled vesicles. Rafts of pumice from undersea volcanoes can float for years and travel across oceans.

Field Identification & Locations

Identify by its extremely light weight, frothy appearance, and ability to float in water. Commonly found in volcanic regions such as the Ring of Fire (USA, Japan, Chile, Iceland). It is often found as rounded pebbles on beaches near volcanic islands.

Identified on: 4/24/2026

Mode: Standard