igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 5-6 (Mohs scale); Color: Dark grey to black, weathering to brown or green; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene; Specific Gravity: 2.8-3.0.

Hardness: 5-6 (Mohs scale)
Color: Dark grey to black, weathering to brown or green
Luster: Dull to sub-metallic

Identified March 16, 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 very near the surface of a terrestrial planet or moon. It is the most common volcanic rock on Earth, covering much of the ocean floor and large continental provinces.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Significant in the industry as 'cast basalt' for wear-resistant liners and used in stone wool insulation.

Geological facts

Basalt is the primary rock type on the lunar maria (the dark spots on the moon) and covers most of the ocean floor. It often forms unique hexagonal columns during cooling, such as those found at the Giant's Causeway in Ireland.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture that lacks visible crystals to the naked eye. It is commonly found in volcanic regions, oceanic islands (like Hawaii), and mid-ocean ridges.

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