Pumice (specifically a water-worn, weathered variety)

Physical Properties

Hardness: 5-6 Mohs (though brittle); Color: Grey/Brown/Black; Luster: Dull/Earthy; Structure: Highly vesicular/porous; Cleavage: None; Specific Gravity: Very low (often floats initially until water-logged).

Formation & Geological History

Formed during explosive volcanic eruptions when highly pressurized, gas-rich lava is ejected. The rapid cooling and depressurization trap gas bubbles as the rock solidifies. This specimen appears weathered/rounded, likely by water transport in a river or beach setting.

Uses & Applications

Used as an abrasive in industrial polishing, as an ingredient in lightweight concrete, and in horticultural soil mixes to improve aeration. Also common in beauty products (exfoliation stones).

Geological Facts

Pumice is the only rock that can float on water when fresh. Following large eruptions, 'pumice rafts' can form in the ocean, lasting for years and traveling thousands of miles across the sea.

Field Identification & Locations

Identify in the field by its sponge-like texture, light weight relative to its size, and the presence of numerous small voids (vesicles). Found in volcanic regions or along coastlines where material has drifted.

Identified on: 4/27/2026

Mode: Standard