igneous

Basalt

Extrusive Mafic Igneous Rock

Hardness: 6 (Mohs scale); Color: Dark gray to black (often weathering to brown or green); Luster: Dull to submetallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8 - 3.0

Hardness: 6 (Mohs scale)
Color: Dark gray to black (often weathering to brown or green)
Luster: Dull to submetallic

Identified March 25, 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 near the surface of a terrestrial planet or moon. Most oceanic crust consists of basalt. Specimens may range from modern to billions of years old.

Uses & applications

Used extensively in construction as road base, railroad ballast, and concrete aggregate. It is also used in the manufacturing of stone wool insulation and occasionally as decorative stone or in 'hot stone' therapies.

Geological facts

Basalt is the most common rock type in the Earth's crust and also covers large areas of the Moon (the Lunar Maria), Mars, and Venus. Famous formations include the Giant's Causeway in Northern Ireland and the Devils Postpile in California.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and lack of visible large crystals unless it is porphyritic (containing larger phenocrysts). Commonly found in volcanic regions, oceanic islands, 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