Pumice

Physical Properties

Hardness: 5-6 (but very brittle); Color: white, cream, or light grey; Luster: dull to pearly; Structure: highly vesicular (full of gas bubbles); Specific Gravity: very low (often floats on water); Texture: frothy or glass-like

Formation & Geological History

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is violently ejected. The rapid cooling and depressurization cause gases to expand and create a frothy texture before the lava solidifies into glass.

Uses & Applications

Used as an abrasive in toothpaste, soaps, and beauty products (pumice stones). Also used in lightweight concrete, filtration systems, and as a horticultural soil amendment.

Geological Facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged. Large eruptions can create 'pumice rafts' in the ocean that can drift for thousands of miles and span miles across.

Field Identification & Locations

Identify in the field by its sponge-like appearance and by checking if it feels surprisingly light for its size. It is common in areas with silicic volcanism, such as the American Northwest, Italy, and Greece.

Identified on: 4/17/2026

Mode: Standard