igneous

Basalt

Mafic Igneous Rock (primarily Plagioclase Feldspar and Pyroxene)

Hardness: 6 (Mohs scale), Color: Dark gray to black (often weathering to brown or rust), Luster: Dull to sub-metallic, Structure: Aphanitic (fine-grained), Cleavage: Absent, Specific Gravity: 2.8–3.0

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Found in oceanic crust, volcanic islands, and continental flood basalts spanning many geological eras.

Uses & applications

Used extensively in construction as road base, railroad ballast, and concrete aggregate. High-density varieties are used for coastal protection (riprap) and stone wool insulation.

Geological facts

Basalt is the most common rock type in the Earth's crust and also covers vast areas of the Moon's surface (the lunar Maria) and Mars.

Field identification & locations

Identify in the field by its heavy weight (density), dark color, and fine-grained texture. It often shows small holes called vesicles or a conchoidal fracture pattern. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.

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