igneous

Pumice

Pumice (Vesicular Rhyolite/Andesite)

Hardness: 5-6 (but brittle and crumbly); Color: Light gray, tan, or cream; Luster: Pearly to dull; Structure: Highly vesicular (foamy) glass; Specific Gravity: less than 1.0 (floats in water).

Hardness: 5-6 (but brittle and crumbly)
Color: Light gray, tan, or cream
Luster: Pearly to dull

Identified March 14, 2026More igneous →

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Physical properties

Hardness: 5-6 (but brittle and crumbly); Color: Light gray, tan, or cream; Luster: Pearly to dull; Structure: Highly vesicular (foamy) glass; Specific Gravity: less than 1.0 (floats in water).

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid cooling and depressurization trap gas bubbles as the rock solidifies. Common in volcanic arcs and subduction zones.

Uses & applications

Used in construction (lightweight concrete/abrasives), cosmetics (exfoliants), textiles (stone-washing denim), and as a soil amendment in horticulture for drainage.

Geological facts

Pumice is the only rock that can reliably float on water until it becomes waterlogged. During massive eruptions, 'pumice rafts' can form on the ocean that are miles long and visible from space.

Field identification & locations

Identify in the field by its incredibly low weight (light as a sponge) and rough, sandpaper-like texture. Commonly found in regions with historical volcanic activity like Italy, Greece, the Pacific Northwest, and Iceland.

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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