igneous

Basalt

Basalt (composition of plagioclase, pyroxene, and olivine)

Hardness: 6 (Mohs scale), Color: Dark grey to black or brownish-grey, Luster: Dull to sub-metallic (if mineral grains visible), Crystal Structure: Aphanitic (fine-grained), Specific Gravity: 2.8–3.0

Identified April 11, 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 moon. Most basalt on Earth is formed by decompression melting of the mantle at mid-ocean ridges.

Uses & applications

Extensively used in construction for aggregate, road base, concrete, and railroad ballast. Exceptionally durable varieties are used for floor tiles and sculpture.

Geological facts

Basalt is the most common rock type in the Earth's crust; almost all of the ocean floor is basalt. It is also common on the Moon, Mars, and Venus in the form of 'maria' or vast plains.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows small vesicles (gas bubbles) or phenocrysts of olivine that appear as translucent green spots.

Other igneous specimens

See all

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral