igneous

Basalt

Basalt (Aphanitic Mafic Igneous Rock)

Hardness: 6 on Mohs scale. Color: typically dark gray to black, turning brownish-red when weathered. Luster: Dull to sub-metallic. Crystal Structure: Fine-grained (aphanitic) due to rapid cooling. Cleavage: None (conchoidal fracture). Specific Gravity: 2.8–3.0.

Hardness: 6 on Mohs scale
Luster: Dull to sub-metallic

Identified March 22, 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 moon. Most terrestrial basalt formed during the Cenozoic era, though it can be found across all geological ages in volcanic regions.

Uses & applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Can be polished for use as floor tiles or architectural stone. Occasionally used for 'hot stone' massage therapy.

Geological facts

Basalt is the most common rock type in Earth's crust; it makes up most of the ocean floor and large provinces on land called flood basalts. It is also found on the Moon, Mars, and Venus.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and lack of visible crystals. It is commonly found in volcanic areas, oceanic islands (like Hawaii), and mid-ocean ridges. Collectors look for vesicles (holes) which may contain secondary minerals like quartz or zeolites.

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