igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0. This specimen appears weathered and water-worn.

Hardness: 6
Color: Dark grey to black
Luster: Dull to sub-metallic

Identified March 13, 2026More igneous →

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Physical properties

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. Most basalt flows are from the Cenozoic era or younger, though they occur throughout geological history.

Uses & applications

Used primarily in construction (crushed stone for road base, concrete aggregate, and railroad ballast), as thermal insulation in the form of stone wool, and occasionally for high-durability floor tiles or decorative landscaping.

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, Mars, and Venus. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt joints.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows small vesicles (gas bubbles) or olivine crystals. Commonly found in volcanic regions, near oceanic ridges, and as river-worn cobbles.

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