igneous

Basalt

Basalt (Mafic Extrusive)

Hardness: 6.0 on Mohs scale; Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0

Hardness: 6
Color: Dark gray to black
Luster: Dull to sub-metallic

Identified May 31, 2026More igneous →

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Physical properties

Hardness: 6.0 on Mohs scale; Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It is typically of Cenozoic or Mesozoic age in many surface locations.

Uses & applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Can be polished for use in floor tiles or counter tops.

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 common on the Moon and Mars.

Field identification & locations

Identify in the field by its dark color, fine grain, and weight (higher density than many other common rocks). Found in volcanic regions like Hawaii, Iceland, and the Columbia River Plateau.

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