igneous

Pumice

Extrusive volcanic rock (Vesicular Rhyolite/Dacite/Andesite)

Hardness: 5-6 Mohs (highly brittle); Color: Pale white, cream, or light grey; Luster: Dull to pearly; Crystal structure: Amorphous/Glassy; Cleavage: None; Specific gravity: 0.25-0.7 (floats on water until saturated)

Hardness: 5-6 Mohs (highly brittle)
Color: Pale white, cream, or light grey
Luster: Dull to pearly

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected into the atmosphere. The rapid cooling and depressurization cause gases to expand rapidly, creating a 'frothy' glass texture full of vesicles (air bubbles).

Uses & applications

Abrasives (pumice stones, exfoliating soaps), construction (lightweight concrete, cinder blocks), horticulture (soil aeration and drainage), and denim 'stone-washing'.

Geological facts

Pumice is the only rock that naturally floats on water. Massive 'pumice rafts' can form in the ocean after underwater volcanic eruptions, sometimes drifting for years and thousands of miles.

Field identification & locations

Identify by its extremely lightweight feel, light color, and 'Swiss cheese' porous texture. It is commonly found in areas of recent volcanic activity, such as Italy, Greece, the Pacific Northwest of the USA, and Iceland.

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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

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral