igneous

Basalt

Extrusive Mafic Igneous Rock (composed primarily of Plagioclase, Pyroxene, and Olivine)

Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0

Hardness: 6 on the Mohs scale
Color: Dark grey to black
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 low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Basalt is found across all geological ages and makes up most of the ocean floor.

Uses & applications

Extensively used in construction as aggregate for roads, concrete, and railroad ballast. In industry, it is melted to create stone wool insulation. High-quality dense specimens are used for pavers and sculpture.

Geological facts

Basalt is the most common rock type in the Earth's crust. It is also prevalent on other planetary bodies, including the Moon (forming the dark 'maria' or seas), Mars, and Venus.

Field identification & locations

Identify by its dark, heavy feel and fine-grained texture. To identify in the field, look for a dark, uniform surface that doesn't reveal individual crystals to the naked eye; it may show tiny vesicles (gas bubble holes) or a weathered brown outer crust.

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