igneous

Pumice

Pumice (Vesicular felsic extrusive igneous rock)

Hardness: 5-6 Mohs. Color: White, cream, grey, or light tan. Luster: Dull to pearly. Crystal structure: Amorphous (volcanic glass). Cleavage: None. Specific gravity: 0.25 to 0.70 (floats in water). Highly vesicular texture with numerous small cavities.

Hardness: 5-6 Mohs
Color: White, cream, grey, or light tan
Luster: Dull to pearly

Identified March 15, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich silicate lava is rapidly cooled. The sudden depressurization allows gas bubbles to expand quickly, creating a frothy structure that solidifies as glass. Usually associated with Cenozoic to recent volcanic activity.

Uses & applications

Used as an abrasive in polishing, skin exfoliation (pumice stones), and heavy-duty cleaners. In construction, it is used for lightweight concrete and breeze blocks. Also popular in horticulture as a soil amendment to improve drainage and aeration.

Geological facts

Pumice is the only rock that can float on water due to its incredible porosity. During massive eruptions like Krakatoa, 'rafts' of floating pumice were reported to be miles long and several feet thick, creating temporary hazards for ships.

Field identification & locations

Identify in the field by its extreme lightness (low density) and abrasive, sandpaper-like feel. It is commonly found in areas of past or present explosive volcanism, such as the Ring of Fire (Japan, Indonesia, USA West Coast) and the Mediterranean.

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Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

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Magnetite

Magnetite (Fe3O4)

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