igneous

Pumice

Pumice (Vitreous pyroclastic rock)

Hardness: 5-6 Mohs (though brittle); Color: White, cream, or light grey; Luster: Dull to pearly; Structure: Vesicular (highly porous/bubbly); Specific Gravity: 0.25–0.9 (floats on water).

Hardness: 5-6 Mohs (though brittle)
Color: White, cream, or light grey
Luster: Dull to pearly

Identified April 7, 2026More igneous →

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Physical properties

Hardness: 5-6 Mohs (though brittle); Color: White, cream, or light grey; Luster: Dull to pearly; Structure: Vesicular (highly porous/bubbly); Specific Gravity: 0.25–0.9 (floats on water).

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 inside the solidifying glass.

Uses & applications

Used as an abrasive in polishing, pencil erasers, and cosmetic exfoliating stones. Also used in lightweight concrete, horticulture (soil aeration), and 'stone-washed' denim production.

Geological facts

Pumice is the only rock that can float on water until it becomes waterlogged. After the 1883 eruption of Krakatoa, rafts of pumice floated across the Indian Ocean for up to 20 years.

Field identification & locations

Identify by its extremely light weight and porous 'sponge-like' texture. Look for it in volcanic regions or wash-up zones on beaches near active or extinct volcanic arcs.

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