igneous

Basalt

Extrusive mafic igneous rock

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene; Specific gravity: 2.8 - 3.0

Hardness: 6 (Mohs scale)
Color: Dark gray to black
Luster: Dull to earthy

Identified May 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Basalt is the most common rock in Earth's crust, forming the bulk of the ocean floor and volcanic islands.

Uses & applications

Extensively used in construction for crushed stone, road base, concrete aggregate, and railroad ballast. Some high-density varieties are used in 'stone wool' insulation or as polished floor tiles.

Geological facts

Basalt covers more than 70% of Earth's surface and also makes up the vast majority of the lunar maria on the Moon. It is the primary rock type produced at mid-ocean ridges.

Field identification & locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture. It often breaks into angular fragments. Commonly found in volcanic regions, oceanic basin edges, and as rounded river rocks in glacial deposits.

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