igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6 (Mohs); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0

Hardness: 6 (Mohs)
Color: Dark gray to black
Luster: Dull to sub-metallic

Identified January 12, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. Common in oceanic crust and volcanic regions.

Uses & applications

Used extensively in construction as road base, concrete aggregate, and railroad ballast. Also used as dimension stone and in high-tensile basalt fibers.

Geological facts

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

Field identification & locations

Identify by its dark color, heavy weight (density), and fine-grained texture. To identify this specific specimen, notice the triangular 'arrowhead' shape which suggests it was worked by humans (lithic artifact).

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