Igneous

Pumice

Vesicular volcanic glass (primarily SiO2 with Al2O3)

Hardness: 5–6 (Mohs), though brittle. Color: White, cream, gray, or light brown. Luster: Vitreous to dull. Structure: Amorphous (non-crystalline) glass. Characteristics: Extensively vesicular (bubbly), extremely low density (often floats in water), rough/scaly texture.

Hardness: 5–6 (Mohs), though brittle
Color: White, cream, gray, or light brown
Luster: Vitreous to dull

Identified January 31, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich silicic magma is ejected into the atmosphere. The rapid cooling and depressurization trap gas bubbles as the magma solidifies into glass. It can be found in deposits dating from the Cenozoic to the present.

Uses & applications

Used as an abrasive in polishing, pencil erasers, and cosmetic exfoliating stones (pumice stones). In construction, it is used as a lightweight aggregate for concrete and breeze blocks. Also used in horticulture for soil aeration and in the 'stone-washing' of denim.

Geological facts

Pumice is the only rock that can reliably float on water for long periods due to its high porosity. Large 'pumice rafts' following underwater eruptions can drift across oceans for years, sometimes carrying biological organisms to new islands.

Field identification & locations

Identify in the field by its sponge-like appearance and incredibly light weight. Common in volcanic regions such as the Ring of Fire, Italy, and Iceland. If a stone feels 'weightless' for its size and has visible holes (vesicles) while being light-colored, it is likely pumice.

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Fluorapatite (Ca5(PO4)3F)

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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.

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Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

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