igneous

Basalt

Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)

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

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

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 exposed at or nearby the surface of a terrestrial planet or a moon. Most common rock type in the Earth's crust (oceanic crust).

Uses & applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used for acid-resistant laboratory equipment and as a base for stone wool insulation.

Geological facts

Basalt is the most common rock type on the surfaces of Mercury, Venus, the Moon, and Mars. Giant's Causeway in Northern Ireland is a famous example of columnar basalt jointing.

Field identification & locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture with no visible crystals to the naked eye. Commonly found in volcanic regions and ocean floor outcrops. Collectors look for vesicles or 'amygdules' filled with secondary minerals like zeolites.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral