igneous

Basalt

Extrusive mafic igneous rock

Hardness: 6.0 on Mohs scale; Color: dark grey to black; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) to vesicular; Cleavage: none (conchoidal fracture); Specific gravity: 2.8–3.0.

Hardness: 6
Color: dark grey to black
Luster: dull to earthy

Identified March 18, 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. Found in oceanic crust, volcanic islands, and flood basalt plateaus. Age varies from modern eruptions to billions of years old.

Uses & applications

Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Used in industry for making rock wool insulation and as high-strength paving stones.

Geological facts

Basalt is the most common rock on Earth's surface and also makes up most of the surface of the Moon, Mars, and Venus. Large hexagonal columns of basalt can form during cooling, such as at the Giant's Causeway.

Field identification & locations

Identify in the field by its dark color, fine grain (crystals are too small to see with the naked eye), and heavy weight for its size. Commonly found in former volcanic regions or near modern fault lines.

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