igneous

Basalt

Extrusive mafic rock (Basalt)

Hardness: 6 (Mohs scale); Color: Dark grey to black with possible weathering to brown; Luster: Dull to somewhat vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black with possible weathering to brown
Luster: Dull to somewhat vitreous

Identified May 3, 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. This specimen appears water-worn from a river or beach environment.

Uses & applications

Used primarily in construction for road base, concrete aggregate, and railroad ballast. Compact, weathered basalt can also be used in river-stone landscaping or as a lapidary material for polishing.

Geological facts

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

Field identification & locations

Identify by its dark color, heavy weight (density), and fine-grained texture which lacks visible crystals. Commonly found along coastlines, volcanic islands, and oceanic ridges.

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