igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6 on Mohs scale. Color: Dark grey to black. Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained), often containing microcrystals of plagioclase feldspar and pyroxene. Cleavage: None (conchoidal or irregular fracture). Specific gravity: 2.8 - 3.0.

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

Identified February 21, 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 surface of a terrestrial planet or moon. It is the most common rock type in the Earth's crust, primarily forming the ocean floor and volcanic islands. Most specimens range from recent to billions of years old.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used as dimension stone for tiles and floorings, and in the production of stone wool insulation.

Geological facts

Basalt covers approximately 70% of the Earth's surface and is also the primary rock found on the Moon (the dark 'Maria' plains) and Mars. Famous basaltic formations include the Giant's Causeway in Northern Ireland and the Devils Postpile in California.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture that lacks visible crystals. It typically breaks with sharp, curved (conchoidal) edges. It is found globally in volcanic regions and oceanic crust exposures. Collectors look for vesicular basalt (containing gas bubbles) or basalt with olivine inclusions.

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