igneous

Basalt

Extrusive Mafic Igneous Rock

Hardness: 6 (Mohs scale); Color: Dark gray to black (often with reddish-brown oxidation); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Cleavage: None (conchoidal fracture); Specific gravity: 2.8-3.0.

Hardness: 6 (Mohs scale)
Color: Dark gray to black (often with reddish-brown oxidation)
Luster: Dull/Earthy

Identified March 29, 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 volcanic provinces and mid-ocean ridges worldwide. Age varies by location, from modern eruptions to billions of years old.

Uses & applications

Used extensively in construction as road base, concrete aggregate, railroad ballast, and high-tensile mineral wool insulation. Smaller polished pieces are used for river stones or aquarium decor.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found in large quantities on the Moon, Mars, and Venus.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows a reddish 'rust' on weathered surfaces due to high iron content. Common in volcanic regions (e.g., Iceland, Hawaii, Pacific Northwest).

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