igneous

Pumice

Pumice (a volcanic glass, primarily SiO2 with Al2O3)

Hardness: 5-6 Mohs; Color: Light gray, tan, or creamy white; Luster: Dull to pearly; Crystal structure: Amorphous (non-crystalline glass); Cleavage: None; Specific gravity: 0.25 - 0.7 (floats on water); Texture: Highly vesicular with frothy bubble-like cavities.

Hardness: 5-6 Mohs
Color: Light gray, tan, or creamy white
Luster: Dull to pearly

Identified January 24, 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 depressurization causes gas bubbles to expand quickly (like opening a soda), and the magma cools so rapidly it hardens into glass before the gas can escape.

Uses & applications

Used as an abrasive in soaps (Lava soap), exfoliating stones for skincare, dental polishing pastes, lightweight aggregate in concrete, and 'stone-washed' denim processing. Also used in horticulture to aerate soil.

Geological facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large eruptions can create 'pumice rafts' in the ocean that can be hundreds of miles long and float for years.

Field identification & locations

Identify in the field by its extreme lightness and 'sponge-like' appearance. It feels abrasive to the touch. It is commonly found in volcanic regions, particularly near rhyolitic or dacitic volcano sites such as those in the Ring of Fire or the Mediterranean.

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