igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 5-6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.

Hardness: 5-6 (Mohs scale)
Color: Dark grey to black, weathering to brown
Luster: Dull to sub-metallic

Identified March 7, 2026More igneous →

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Physical properties

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. Most common rock type on Earth's surface, particularly on the ocean floor.

Uses & applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used for building stone, pavers, and as decorative landscaping rock.

Geological facts

Basalt makes up most of the ocean floors and is also found on the Moon, Mars, and Venus. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt joints formed by cooling contraction.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture that lacks visible crystals to the naked eye. Found worldwide in volcanic regions like Hawaii, the Pacific Northwest (Columbia River Basalts), 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