igneous

Basalt

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6 on Mohs scale; Color: typically dark grey to black; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) or porphyritic; Cleavage: none; Specific gravity: 2.8 - 3.0 (dense).

Hardness: 6 on Mohs scale
Color: typically 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 surface of a terrestrial planet or a moon. Most basalt flows are from the Cenozoic era, though it has formed throughout Earth's history.

Uses & applications

Extensively used in construction as an aggregate (crushed stone) for road bases, concrete, and railroad ballast. In industry, it is used for high-tensile basalt fibers and as 'cast basalt' for wear-resistant liners.

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 extensively on the Moon and Mars (the dark plains on the Moon, known as Maria, are basaltic).

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture with no visible crystals to the naked eye. Found worldwide in volcanic regions, such as the Giant's Causeway or the Hawaiian Islands.

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