igneous

Basalt

Basalt (Mafic Extrusive)

Hardness: 6.0 on Mohs scale. Color: Dark grey to black, sometimes greenish. Luster: Dull to sub-metallic when fresh. Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene. Specific gravity: 2.8–3.0.

Hardness: 6
Color: Dark grey to black, sometimes greenish
Luster: Dull to sub-metallic when fresh

Identified April 2, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (magma) at or very near the Earth's surface. It is the most common rock type in Earth's crust, primarily making up the ocean floors.

Uses & applications

Extensively used in construction as aggregate for roads, railroad ballast, and concrete. It is also used as a dimension stone for flooring, monuments, and occasionally in stone wool insulation manufacturing.

Geological facts

Basalt is not only the most common rock on Earth but also on other terrestrial planets like Mars and Venus, and the Moon. The 'Maria' (dark plains) of the Moon are composed of basaltic lava flows.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often weathers to a reddish-brown color due to iron oxidation. Look for columnar jointing in large outcrops.

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