igneous

Basalt

Extrusive Mafic Igneous Rock

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to black
Luster: Dull

Identified January 12, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) at or very near the Earth's surface. It can be found in volcanic oceanic crust, mid-ocean ridges, and continental flood basalts of various geological ages from the Archean to the present.

Uses & applications

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used as dimension stone for tiles and monuments, and spun into rock wool for thermal insulation.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floors. It is also found on other celestial bodies like the Moon (lunar maria), Mars, and Venus.

Field identification & locations

Identify in the field by its high density, dark color, and lack of visible individual crystals without a lens. Look for a fine-grained, 'sugary' texture and occasional small, green olivine crystals. Common in volcanic regions such as Iceland, Hawaii, and the Pacific Northwest of the USA.

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