igneous

Basalt

Basalt (Aphanitic Mafic Igneous Rock)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often weathering to brown or green; Luster: Dull to sub-metallic; Crystal structure: Microcrystalline (aphanitic) or porphyritic; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Hardness: 6-7 on Mohs scale
Color: Dark grey to black, often weathering to brown or green
Luster: Dull to sub-metallic

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic) exposed at or very near the Earth's surface. It can vary in age from modern volcanic flows to billions of years old, often found in oceanic crust and large igneous provinces.

Uses & applications

Extensively used in construction as aggregate for roads, railroad ballast, and concrete. It is also used as a dimension stone for floor tiles, monuments, and 'crushed stone' in landscaping. Dense varieties are used as a raw material for stone wool insulation.

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 (the dark lunar Maria) and Mars. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt.

Field identification & locations

Identify in the field by its dark color, fine-grained texture (individual crystals are too small to see with the naked eye), and high density. It is commonly found in volcanic regions, along mid-ocean ridges, and in areas with ancient lava flows. Collectors look for vesicles (air bubbles) containing secondary minerals like zeolites 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