igneous

Pumice

Pumice (Alveolar Rhyolite / Dacite / Andesite)

Hardness: 5-6 Mohs (though very brittle); Color: white, cream, tan, or gray; Luster: Pearly to dull; Structure: Highly vesicular (honeycombed with air bubbles or vesicles); Cleavage: None; Specific Gravity: 0.25 to 0.6 (typically floats in water).

Hardness: 5-6 Mohs (though very brittle)
Color: white, cream, tan, or gray
Luster: Pearly to dull

Identified May 4, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected rapidly, cools quickly, and depressurizes, allowing gas bubbles to expand and freeze in place. This creates a frothy glass texture.

Uses & applications

Used as an abrasive in soaps (like Lava soap), beauty treatments (pumice stones for feet), stone-washing denim, lightweight concrete production, and horticultural soil drainage.

Geological facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large rafts of floating pumice can drift across oceans for years after an undersea eruption.

Field identification & locations

Identify in the field by its extreme lightness (it will feel like Styrofoam) and its numerous tiny holes (vesicles). It is commonly found near volcanic sites such as the Cascades in the US or volcanic islands.

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