igneous

Pumice

Highly vesicular volcanic glass (unnamed scientific variant: Rhyolitic/Andesitic Pumice)

Hardness: 5-6 (but brittle). Color: Light gray, white, or cream. Luster: Dull to pearly. Structure: Highly vesicular/porous (glassy froth). Specific Gravity: Often < 1.0 (floats in water).

Hardness: 5-6 (but brittle)
Color: Light gray, white, or cream
Luster: Dull to pearly

Identified March 19, 2026More igneous →

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Physical properties

Hardness: 5-6 (but brittle). Color: Light gray, white, or cream. Luster: Dull to pearly. Structure: Highly vesicular/porous (glassy froth). Specific Gravity: Often < 1.0 (floats in water).

Formation & geological history

Formed during explosive volcanic eruptions when lava is ejected into the air. Dissolved gases expand rapidly as pressure drops, creating a foam that cools and solidifies before the bubbles can escape. Occurs globally in active and extinct volcanic regions.

Uses & applications

Used as an abrasive in beauty products (pumice stones), heavyweight-reducing concrete aggregate, dental polishing paste, and as a soil amendment in horticulture to improve drainage and aeration.

Geological facts

Pumice is the only rock that can float on water due to its air-filled vesicles. Ancient Romans used pumice in the construction of the Pantheon's dome to reduce the structural weight. Following large eruptions, 'pumice rafts' can float on the ocean for years.

Field identification & locations

Identify in the field by its extreme lightness relative to its size and its porous, sponge-like appearance. It is commonly found in ash fall deposits or near volcanic vents. Collect from areas with felsic volcanic activity like Oregon, California, or Italy.

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