igneous

Basalt

Extrusive Mafic Igneous Rock

Hardness: 6 (Mohs scale), Color: Dark gray to black, Luster: Dull/Earthly, Crystal structure: Aphanitic (fine-grained), Cleavage: None/Fragmental, Specific gravity: 2.9-3.1

Identified May 7, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale), Color: Dark gray to black, Luster: Dull/Earthly, Crystal structure: Aphanitic (fine-grained), Cleavage: None/Fragmental, Specific gravity: 2.9-3.1

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Found in oceanic crust, volcanic islands, and flood basalt provinces.

Uses & applications

Extensively used in construction, road base, concrete aggregate, railroad ballast, and ground for high-quality insulation (stone wool).

Geological facts

Basalt is the most common rock type in the Earth's crust and almost the entirety of the ocean floor is composed of it. It is also common on the Moon and Mars.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture lacking visible crystals without a lens. Found globally in volcanic regions.

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