igneous

Pumice

Pumiceous rhyolite / Volcanic glass

Mohs hardness: 5-6; Color: Light gray, cream, or off-white; Luster: Dull to pearly; Structure: Vesicular (highly porous/holey); Cleavage: None; Specific gravity: 0.25-0.90 (floats in water).

Hardness: 5-6
Color: Light gray, cream, or off-white
Luster: Dull to pearly

Identified April 9, 2026More igneous →

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Physical properties

Mohs hardness: 5-6; Color: Light gray, cream, or off-white; Luster: Dull to pearly; Structure: Vesicular (highly porous/holey); Cleavage: None; Specific gravity: 0.25-0.90 (floats in water).

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich molten rock (magma) is ejected. The rapid cooling and depressurization trap gas bubbles as the glass solidifies instantly.

Uses & applications

Used as an abrasive in soaps (pumice stones) and toothpastes, in horticulture as a soil amendment, in lightweight concrete blocks, and for stone-washing denim jeans.

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' that drift for thousands of miles across the ocean.

Field identification & locations

Identify in the field by its extremely light weight and porous 'sponge-like' appearance. It is commonly found in areas of past or present volcanic activity, such as Italy, Turkey, and the Western USA.

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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