igneous

Pumice

Pumice (a volcanic glass, primarily SiO2 with Al2O3)

Hardness: 5-6 Mohs (though brittle); Color: White, cream, or light grey (occasionally pinkish); Luster: Dull/pearly; Structure: Vesicular (highly porous/bubbly); Specific Gravity: 0.25-0.9 (floats in water).

Hardness: 5-6 Mohs (though brittle)
Color: White, cream, or light grey (occasionally pinkish)
Luster: Dull/pearly

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when pressurized lava is violently ejected. The rapid cooling and depressurization trap gas bubbles in a frothy silica-rich glass. It can be found in volcanic regions of all geological ages, notably Cenozoic.

Uses & applications

Used as a lightweight aggregate in concrete, as an abrasive in 'pumice stones' for beauty treatments, in pencil erasers, stone-washed jeans processing, and horticulture to improve soil aeration.

Geological facts

Pumice is the only rock that consistently floats on water. Massive 'pumice rafts' created by underwater eruptions can drift across oceans for years, sometimes reaching thousands of miles in length.

Field identification & locations

Identify by its extremely low weight and frothy, 'sponge-like' appearance. Found in volcanic areas like the Andes, Italy, Greece, and the Cascades. Collectors should look for specimens with exceptionally large vesicles or unique coloration.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral