igneous

Basalt

Extrusive mafic igneous rock

Hardness: 6.0 (Mohs scale); Color: Dark grey, black, or dark greenish-grey; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0

Hardness: 6
Color: Dark grey, black, or dark greenish-grey
Luster: Dull to sub-metallic

Identified March 25, 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. It can be found in oceanic crust and via volcanic activity throughout geological history.

Uses & applications

Used extensively in construction as crushed stone for road bases, concrete aggregate, railroad ballast, and high-quality asphalt. Also used in stone wool insulation and occasionally as decorative stone.

Geological facts

Basalt is the most common rock type in the Earth’s crust and makes up most of the ocean floors. It is also common on other terrestrial planets like Mars and Venus, and even our Moon.

Field identification & locations

Identify in the field by its high density, dark color, and fine-grained texture with occasional small green (olivine) or white (plagioclase) crystals. Found widely 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