Igneous

Pumice

Pumice (a highly vesicular extrusive igneous rock)

Hardness: 5-6 Mohs; Color: Light gray, tan, or cream; Luster: Pearly to dull; Structure: Vesicular (gas-filled bubbles), amorphous (glassy); Specific Gravity: 0.25 - 0.7 (often floats in water).

Hardness: 5-6 Mohs
Color: Light gray, tan, or cream
Luster: Pearly to dull

Identified March 22, 2026More igneous →

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Physical properties

Hardness: 5-6 Mohs; Color: Light gray, tan, or cream; Luster: Pearly to dull; Structure: Vesicular (gas-filled bubbles), amorphous (glassy); Specific Gravity: 0.25 - 0.7 (often floats in water).

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich rhyolitic or andesitic lava is violently ejected. The rapid cooling and depressurization trap gas bubbles, creating a porous glass foam.

Uses & applications

Used as an abrasive in polishing and cleaning, as a lightweight aggregate in concrete/cinder blocks (horticulture), and in personal care as a pumice stone for exfoliation.

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 are hundreds of miles long.

Field identification & locations

Identify by its light weight (low density) and the presence of numerous tiny pits or vesicles. Commonly found in volcanic regions, along shorelines after eruptions, or in decorative landscaping gravel.

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