igneous

Pumice

Pumice (Highly vesicular felsic extrusive igneous rock; primarily SiO2)

Hardness: 5-6 (Mohs), but brittle; Color: Light gray, white, or cream; Luster: Pearly to dull; Structure: Highly vesicular (foamy) with glass-walled bubbles; Specific Gravity: Below 1.0 (often floats in water).

Hardness: 5-6 (Mohs), but brittle
Color: Light gray, white, or cream
Luster: Pearly to dull

Identified March 19, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich molten rock (magma) is violently ejected. The rapid cooling and depressurization trap gas bubbles in a glass matrix. It is found in volcanic regions of all geological ages, especially near subduction zones.

Uses & applications

Used as a lightweight aggregate in concrete/cinder blocks, a decorative landscaping stone, an abrasive in 'stone-washed' jeans, and in personal care for exfoliating skin and polishing teeth.

Geological facts

Pumice is the only rock that can float on water due to its extreme porosity. After major volcanic eruptions, massive 'pumice rafts' can form on the ocean, some of which are large enough to be seen from space and can float for years.

Field identification & locations

Identify by its light weight and rough, scratchy texture; it feels almost like a solidified sponge. Common in locations like the Aegean Islands, the American West (Cascades), and Italy. Collectors look for pieces with unique colors or distinct fiber-like structures.

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