Pumice

Physical Properties

Hardness: 5-6 (Mohs), Color: Light gray to white/cream, Luster: Dull to pearly, Crystal structure: Amorphous (volcanic glass), Cleavage: None, Specific gravity: 0.25-1.2 (often floats on water)

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) within the rock. This can occur in underwater or terrestrial volcanic environments.

Uses & Applications

Used as an abrasive in soaps (lava soap), skincare (pumice stones), polishing compounds, and lightweight concrete or breeze blocks in construction. Also used in horticulture as a soil amendment.

Geological Facts

Pumice is the only rock that can float on water until it becomes waterlogged. Large eruptions can create 'pumice rafts' that drift across oceans for thousands of miles, transporting various sea life to new ecosystems.

Field Identification & Locations

Look for a very lightweight, frothy, or porous texture. It is often pale in color and feels rough or abrasive to the touch. It is found in volcanic regions like the Cascades (USA), Italy, and New Zealand.

Identified on: 4/28/2026

Mode: Standard