Pumice

Physical Properties

Hardness: 5-6 (but very brittle); Color: Cream, light gray, or tan; Luster: Dull to pearly; Structure: Highly vesicular (honeycombed with air bubbles); Specific Gravity: 0.25 to 0.7 (floats on water).

Formation & Geological History

Formed during explosive volcanic eruptions when highly pressurized, gas-charged lava is ejected. The rapid cooling and depressurization trap gas bubbles, creating a solidified foam. Most specimens are relatively young geologically (Cenozoic).

Uses & Applications

Used as an abrasive in cleaning products and skin exfoliants, as a lightweight aggregate in concrete/cinder blocks, and in stonewashing denim jeans.

Geological Facts

Pumice is the only rock that can float on water for long periods, sometimes forming large 'pumice rafts' in the ocean that can travel thousands of miles after an eruption.

Field Identification & Locations

Identify by its extremely lightweight feel and visible pores. Look for it near volcanic regions or on beaches where it has washed ashore. It will scratch glass but is easily crushed.

Identified on: 4/20/2026

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