igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6.0 on Mohs scale; Color: dark grey to black; Luster: dull to sub-metallic; Crystal Structure: fine-grained/aphanitic; Specific Gravity: 2.8 - 3.0

Hardness: 6
Color: dark grey to black
Luster: dull to sub-metallic

Identified March 6, 2026More igneous →

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Physical properties

Hardness: 6.0 on Mohs scale; Color: dark grey to black; Luster: dull to sub-metallic; Crystal Structure: fine-grained/aphanitic; Specific Gravity: 2.8 - 3.0

Formation & geological history

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic) at or very near the Earth's surface. Common in oceanic crust and continental flood basalt provinces.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as dimension stone and for stone wool insulation.

Geological facts

Basalt is the most common rock type in Earth's crust and also covers large areas of the Moon, Mars, and Venus. The famous Giant's Causeway in Ireland is made of basaltic columns.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture with no visible individual crystals. Common in volcanic regions and desert pavements as desert varnish.

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