igneous

Basalt

Extrusive mafic igneous rock

Hardness: 6.0; Color: dark gray to black, often weathering to brown; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; Specific gravity: 2.8 - 3.0

Hardness: 6
Color: dark gray to black, often weathering to brown
Luster: dull to sub-metallic

Identified April 12, 2026More igneous →

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Physical properties

Hardness: 6.0; Color: dark gray to black, often weathering to brown; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; 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 Earth's surface. Basalt is the most common volcanic rock on Earth and forms much of the ocean floor and large continental provinces.

Uses & applications

Extensively used in construction for road base, concrete aggregate, railroad ballast, and high-quality asphalt. It is also used in decorative landscaping and as 'basalt fiber' for thermal insulation.

Geological facts

Basalt is also found on other planetary bodies, including the Moon, Mars, and Venus. On Earth, the Giant's Causeway in Northern Ireland is a famous example of columnar basalt jointing.

Field identification & locations

Identify by its heavy weight (high density), dark color, and lack of visible large crystal grains without a lens. Look for a 'vesicular' texture (small holes from gas bubbles) or a smooth, dense fracture surface.

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