igneous

Pumice

Pumice (a highly vesicular volcanic glass)

Hardness: 5-6 (but very brittle); Color: Light gray, white, or cream; Luster: Dull to pearly; Crystal structure: Amorphous/Vitreous; Cleavage: None; Specific Gravity: Below 1.0 (floats on water); Texture: Vesicular (full of tiny holes).

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

Identified April 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected and cools rapidly. The decompression creates bubbles (vesicles) trapped in the solidifying glass. Age can vary from modern Holocene to millions of years old.

Uses & applications

Used as an abrasive in cleaning (pumice stones), in skincare (exfoliation), in lightweight concrete, as a filtration medium, and in 'stone-washing' denim for apparel.

Geological facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large 'pumice rafts' can form on the ocean after underwater volcanic eruptions and drift for thousands of miles.

Field identification & locations

Identify by its extreme lightness (low density), light color, and 'sponge-like' texture with many visible holes. It won't scratch easily with a fingernail but will float. Commonly found near active or dormant explosive volcanoes (e.g., Cascades, Mediterranean).

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

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