Igneous

Basalt

Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)

Hardness: 6.0 on Mohs scale; Color: Dark grey to black, weathering to rust-brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.

Hardness: 6
Color: Dark grey to black, weathering to rust-brown
Luster: Dull to sub-metallic

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic magma) at or very near the Earth's surface. Basaltic crust makes up most of the ocean floors and is found in vast 'traps' or flood basalt provinces across all geological ages.

Uses & applications

Extensively used in construction as aggregate for road base, concrete, and railroad ballast. Also used as dimension stone (tiles), in the manufacturing of stone wool insulation, and occasionally as structural 'basalt fiber' in engineering.

Geological facts

Basalt is the most common rock type in Earth's crust and also covers large areas of the Moon, Mars, and Venus. On the Moon, the dark areas known as 'maria' are actually vast basaltic plains formed by ancient volcanic eruptions.

Field identification & locations

Identify in the field by its heavy weight (density), dark uniform color, and very fine grains that are usually too small to see with the naked eye. It often breaks into sharp fragments or forms hexagonal 'columnar joints' in large outcrops. Common in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

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