igneous

Basalt Pebble

Extrusive mafic igneous rock

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull or earthy when weathered; Structure: Fine-grained (aphanitic), sometimes vesicular; Cleavage: None/fractional; Density: 2.8–3.0 g/cm³.

Hardness: 6 (Mohs scale)
Color: Dark grey to black
Luster: Dull or earthy when weathered

Identified April 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic) at or very near the Earth's surface. This specimen has been rounded and smoothed by water erosion in a river or coastal environment.

Uses & applications

Commonly used in construction for road base, concrete aggregate, and railroad ballast. Smooth pebbles are popular for landscaping, aquarium decoration, and 'hot stone' massage therapy.

Geological facts

Basalt is the most common rock type in Earth's crust and makes up the majority of the ocean floor. It is also found extensively on the Moon and Mars.

Field identification & locations

Identify by its dark color, heavy weight for its size, and absence of visible large crystals. Commonly found on beaches or in stream beds near volcanic regions.

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