igneous

Basalt

Mafic Igneous Rock

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

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

Identified January 12, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull or 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 exposed at or very near the surface of a terrestrial planet or a moon. It represents the most common volcanic rock on Earth.

Uses & applications

Extensively used in construction for aggregate in concrete and asphalt, road base, railroad ballast, and as high-quality floor tiles or building stone.

Geological facts

Basalt is the primary rock type of the ocean floor and is also found on the Moon and Mars. On Earth, it often forms distinct hexagonal columns during cooling, known as columnar jointing.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture that lacks visible crystals. It is ubiquitous in volcanic regions such as Iceland, Hawaii, and the 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