igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 5-6 Mohs; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained), often containing micro-crystals of plagioclase feldspar and pyroxene; Specific Gravity: 2.8-3.0.

Hardness: 5-6 Mohs
Color: Dark grey to black
Luster: Dull to sub-metallic

Identified March 30, 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. Most terrestrial basalt formed during the Cenozoic era, though it can date back billions of years.

Uses & applications

Extensively used in construction as aggregate (crushed stone) for road base, concrete, and asphalt; used in industrial high-temp insulation (rock wool); occasionally polished for decorative river stones and jewelry.

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 the primary rock type found on the Moon (the dark 'seas' or Maria) and Mars.

Field identification & locations

Identify in the field by its heavy weight (density), dark color, and lack of visible large crystals. Commonly found in volcanic regions, oceanic islands (like Hawaii), and riverbeds near volcanic plateaus. Collectors look for vesicles (gas bubbles) or 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