Igneous

Basalt

Mafic Igneous Rock (composed primarily of plagioclase feldspar and pyroxene)

Hardness: 6 (Mohs scale); Color: Dark gray, black, or dark greenish-gray; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Cleavage: Generally lacks cleavage; Specific Gravity: 2.8-3.0.

Hardness: 6 (Mohs scale)
Color: Dark gray, black, or dark greenish-gray
Luster: Dull to sub-metallic

Identified March 30, 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 the most common rock in the Earth's crust and underlies most ocean basins.

Uses & applications

Extensively used as an aggregate in construction for road base, railroad ballast, and concrete. It is also used as dimension stone for tiles and monuments, and finely ground basalt is used as a soil amendment.

Geological facts

Basalt covers approximately 70% of the Earth's surface. It is also the primary rock type on the Moon's maria, Mars, and Venus, making it a truly 'interplanetary' rock.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows columnar jointing or vesicular (pitted) surfaces when it contains trapped gas bubbles. Found in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

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

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