igneous

Pumice

Pumice (a highly vesicular, silicic volcanic rock)

Hardness: 5-6 Mohs; Color: light grey to cream; Luster: dull to pearly; Structure: highly vesicular (frothy glass); Specific Gravity: 0.25-0.9 (often floats on water); Composition: primarily amorphous volcanic glass (SiO2).

Hardness: 5-6 Mohs
Color: light grey to cream
Luster: dull to pearly

Identified March 21, 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 cause gas bubbles to expand quickly, creating a frothy structure that solidifies into glass.

Uses & applications

Used as an abrasive in polishing and cleaning, in lightweight concrete blocks, as a decorative garden stone, and in skincare as an exfoliant for removing dead skin.

Geological facts

Pumice is the only rock that consistently floats on water due to its trapped air pockets. In 1883, the eruption of Krakatoa created pumice rafts that floated on the ocean for years, some traveling across entire oceans.

Field identification & locations

Identify in the field by its extreme lightness (low density) and vesicular, sponge-like surface. It is commonly found near volcanic arcs and subduction zones like the Andes, Japan, or the Cascades.

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