igneous

Basalt

Extrusive mafic igneous rock

Hardness: 5-6 (Mohs scale); Color: dark grey to black; Luster: dull to sub-metallic; Structure: fine-grained (aphanitic) sometimes with visible phenocrysts; Specific Gravity: 2.8-3.0.

Hardness: 5-6 (Mohs scale)
Color: dark grey to black
Luster: dull to sub-metallic

Identified February 26, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs scale); Color: dark grey to black; Luster: dull to sub-metallic; Structure: fine-grained (aphanitic) sometimes with visible phenocrysts; Specific Gravity: 2.8-3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or moon. Most basalt on Earth formed during the Cenozoic era, though it exists across all geological ages.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, railroad ballast, and for high-durability floor tiles.

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.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows a weathered brown exterior but is dark grey inside. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.

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