igneous

Pumice

Vesicular volcanic glass (primarily Alumino-silicates)

Hardness: 5-6 (but brittle), Color: White, cream, grey, or light tan, Luster: Dull to pearly, Structure: Highly vesicular (honeycombed with air bubbles), Specific Gravity: Very low (0.25–1.2), often floats on water.

Identified January 13, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich silicate lava is violently ejected. The rapid cooling and depressurization lock air bubbles into the solidifying glass matrix.

Uses & applications

Used as an abrasive in skincare (exfoliation), heavy-duty cleaning products, polishing compounds for glass and metal, and as a lightweight aggregate in concrete and gardening (soil aeration).

Geological facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged. In a large eruption, 'pumice rafts' can form on the ocean and drift for thousands of miles.

Field identification & locations

Identify in the field by its extreme lightness (low density) and frothy, sponge-like texture. It is commonly found in areas of recent or past explosive volcanic activity, such as Italy, Greece, the American West, and Iceland.

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