igneous

Pumice

Pumice (a highly vesicular extrusive igneous rock, primarily amorphous silica)

Hardness: 6 (Mohs), though brittle; Color: light gray, white, or cream; Luster: dull to pearly; Structure: highly vesicular (frothy) glass; Cleavage: none; Specific Gravity: 0.25 to 0.70 (low enough to float in water)

Hardness: 6 (Mohs), though brittle
Color: light gray, white, or cream
Luster: dull to pearly

Identified March 19, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich silicate magma is ejected into the atmosphere. The rapid cooling and depressurization cause gas bubbles to be trapped within the solidifying rock. Commonly found in subduction zones.

Uses & applications

Used as an abrasive in polishing, exfoliating soaps (pumice stones), and erasers; in construction as a lightweight aggregate for concrete; and in horticultural soil mixes to improve aeration.

Geological facts

Pumice is the only rock that can float on water until it becomes waterlogged. Mass rafts of pumice have been known to float in the ocean for years following major eruptions like Krakatoa or Tonga.

Field identification & locations

Identify in the field by its extreme lightness, sandpaper-like texture, and many small visible cavities (vesicles). It is commonly found near volcanic sites such as the Cascades in the US, Italy, and Greece.

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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.

Igneous

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)

mineral