Igneous

Pumice

Felsic Volcanic Glass / Vesicular Rhyolite

Hardness: 5-6 Mohs (though brittle). Color: light gray, white, or cream. Luster: dull to pearly. Structure: Highly vesicular (honeycombed with gas bubbles), amorphous glass. Specific Gravity: < 1.0 (it floats).

Hardness: 5-6 Mohs (though brittle)
Color: light gray, white, or cream
Luster: dull to pearly

Identified March 19, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when magma with high gas and water content is ejected into the air. The rapid cooling and depressurization trap gas bubbles in the solidifying glass. Occurs across all geological ages, especially near subduction zones.

Uses & applications

Used as an abrasive in polishing, pencil erasers, and skincare (exfoliation). In construction, it is used for lightweight concrete and breeze blocks. Also used in horticulture to aerate soil and in denim 'stone-washing'.

Geological facts

Pumice is the only rock known to float on water until it eventually becomes waterlogged and sinks. Large eruptions can create 'pumice rafts' several kilometers long that can travel across oceans for years.

Field identification & locations

Identify in the field by its extreme lightness (low density) and porous, sponge-like surface. It is commonly found in volcanic regions like the American Northwest, Italy, Greece, and Iceland. Collectors look for pieces with unique inclusions or large pore structures.

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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