igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6-7 on Mohs scale. Color: dark gray to black. Luster: dull to matte. Crystal structure: aphanitic (fine-grained). Specific gravity: 2.8-3.0.

Hardness: 6-7 on Mohs scale
Color: dark gray to black
Luster: dull to matte

Identified May 21, 2026More igneous →

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Physical properties

Hardness: 6-7 on Mohs scale. Color: dark gray to black. Luster: dull to matte. Crystal structure: aphanitic (fine-grained). Specific gravity: 2.8-3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface of a terrestrial planet or moon. It is the most common rock type in the Earth's crust.

Uses & applications

Used primarily in construction for aggregate, road base, railroad ballast, and high-quality concrete. Can be used in jewelry when polished as 'river stones'.

Geological facts

Basalt is the primary rock making up the ocean floors and is also found extensively on the Moon and Mars. It forms hexagonal columns when it cools slowly, like at Giant's Causeway.

Field identification & locations

Identify by its heavy weight (density), dark color, and lack of visible crystals to the naked eye. Often found as rounded river or beach cobbles when weathered.

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