igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6 on Mohs scale; Color: Dark grey to black, often with a weathered brown exterior; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0

Hardness: 6 on Mohs scale
Color: Dark grey to black, often with a weathered brown exterior
Luster: Dull to sub-metallic

Identified May 9, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) at or very near the Earth's surface. It can be found in oceanic crust, volcanic islands, and continental flood basalts.

Uses & applications

Extensively used in construction for aggregate, road base, concrete, and railroad ballast. High-density varieties are used in sea walls. Fine specimens may be used for stonework or as decorative landscape rocks.

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.

Field identification & locations

Identify by its heavy weight (density), dark color, and very fine-grained texture. It often shows a weathered 'rind' that is lighter in color than the fresh interior. Found commonly in volcanic regions such as Hawaii, Iceland, and the Pacific Northwest.

Other igneous specimens

See all

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