igneous

Pumice

Pumice (a highly vesicular felsic volcanic rock)

Hardness: 5-6 (Mohs), but brittle. Color: White, light gray, or cream. Luster: Dull to pearly. Structure: Highly vesicular/porous (frothy). Cleavage: None. Specific Gravity: 0.25-0.9 (floats on water).

Hardness: 5-6 (Mohs), but brittle
Color: White, light gray, or cream
Luster: Dull to pearly

Identified March 15, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid cooling and depressurization trap gas bubbles as the rock solidifies, creating a 'frothy' glass.

Uses & applications

Used as an abrasive in cleaning products (pumice stones), cosmetics (exfoliants), construction (lightweight concrete), and horticulture (soil aeration).

Geological facts

Pumice is the only rock that can float on water due to its extreme porosity. Massive 'pumice rafts' have been known to float in the ocean for years following major volcanic eruptions.

Field identification & locations

Identify in the field by its extreme lightness and pitted, sponge-like appearance; it will feel significantly lighter than average rocks of its size. Common in volcanic regions like the Andes, Cascades, and Italy.

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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