igneous

Basalt (River Cobble)

Basalt (Mafic volcanic rock)

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black (interior), brownish-tan weathering rind (exterior); Luster: Dull/Sub-metallic; Structure: Aphanitic (fine-grained), sometimes vesicular; Cleavage: None.

Hardness: 6 (Mohs scale)
Luster: Dull/Sub-metallic

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been chemically weathered and physically abraded in a river environment, creating a rounded cobble with a distinct oxidation rind.

Uses & applications

Used extensively in construction as road base, aggregate in concrete and asphalt, and railroad ballast. Exceptionally dense specimens are sometimes used for high-quality stone carvings or hot stone massage therapy.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus, indicating widespread volcanic activity across the solar system.

Field identification & locations

Identify in the field by its high density (feels heavy for its size), dark interior when broken, and lack of visible large crystals. Commonly found in volcanic regions or riverbeds downstream from volcanic plateaus. Look for the contrast between the weathered outer 'skin' and the fresh dark interior.

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