igneous

Basalt

Mafic Igneous Rock (composed primarily of Plagioclase and Pyroxene)

Hardness: 5-6 (Mohs scale); Color: dark gray to black; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) sometimes with visible phenocrysts; Cleavage: none; Specific gravity: 2.8-3.0.

Hardness: 5-6 (Mohs scale)
Color: dark gray to black
Luster: dull to sub-metallic

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava at or very near the Earth's surface. Found in tectonic plates and volcanic hotspots; forms the majority of the ocean floor and large continental flood basalts.

Uses & applications

Extensively used in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. Also used as dimension stone for tiles and monuments, or as 'lava rock' in landscaping.

Geological facts

Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon (the Maria) and Mars. Some basalt flows form spectacular hexagonal columns, such as at the Giant's Causeway.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture with little to no quartz visible. Often found near volcanic regions or as road gravel in many areas.

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