igneous

Basalt

Aphanitic mafic volcanic rock (primarily plagioclase feldspar and pyroxene)

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

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

Identified March 18, 2026More igneous →

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Physical properties

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

Formation & geological history

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic) at or very near the Earth's surface. Basaltic volcanism has occurred throughout Earth's history, from billions of years ago to modern active volcanic zones.

Uses & applications

Commonly used as crushed stone for road base, railroad ballast, and concrete aggregate. Sometimes used in masonry and landscaping (lava rock).

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 and Mars.

Field identification & locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture. Found globally in volcanic regions like the Pacific Northwest, Hawaii, and Iceland. Check for small vesicles (gas bubbles) or 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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral