igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0.

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

Identified February 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Common in oceanic crust and hot spot volcanoes. Ages vary from Precambrian to modern-day eruptions.

Uses & applications

Extensively used in construction for road base, concrete aggregate, railroad ballast, and dimension stone for pavers or countertops. High-quality scoria or vesicular basalt is used for landscaping.

Geological facts

Basalt is the most common rock type in Earth's crust and also covers large portions of the surfaces of the Moon, Mars, and Venus. On Earth, giant 'flood basalt' events have created massive formations like the Deccan Traps.

Field identification & locations

Identify in the field by its high density, dark color, and fine-grained texture (often showing small holes called vesicles). Found worldwide in volcanic regions such as Iceland, Hawaii, and the Pacific Northwest. For collectors, look for basalt containing olivine crystals.

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