igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull/Matte, Crystal structure: Aphanitic (fine-grained), Specific gravity: 2.8–3.0

Identified May 18, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull/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 exposed at or very near the surface of a terrestrial planet or a moon. Most common rock in the Earth's crust.

Uses & applications

Extensively used in construction for aggregate in highway engineering, railroad ballast, and tile flooring. Also used in high-performance insulation (mineral wool).

Geological facts

Basalt is the primary rock type on the seafloor and makes up the majority of the lunar maria on the Moon. Some columns of basalt form perfect hexagonal shapes like Giant's Causeway.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows small holes (vesicles) where gas was trapped during cooling.

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