Pumice

Physical Properties

Hardness: 5-6 (but very brittle); Color: Off-white, light gray, or cream; Luster: Dull to pearly; Structure: High vesicularity (porous); Specific Gravity: Sub-1.0 (it often floats on water initially).

Formation & Geological History

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is violently ejected. The rapid cooling and depressurization trap gas bubbles, creating a solidified foam-like rock.

Uses & Applications

Used as a lightweight aggregate in concrete, as an abrasive in industrial polishing, in horticulture to aerate soil, and in beauty products for exfoliation (pumice stones).

Geological Facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged. Massive 'rafts' of floating pumice can be thousands of miles long following volcanic eruptions.

Field Identification & Locations

Identify in the field by its extreme lightness (low density) and sandpaper-like texture. Common in regions with active or historical explosive volcanism like the Cascades, Italy, or Iceland.

Identified on: 4/17/2026

Mode: Standard