Igneous

Basalt

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to vitreous; Crystal Structure: Aphanitic (fine-grained), sometimes porphyritic with larger phenocrysts; Cleavage: None; Specific Gravity: 2.8-3.0

Hardness: 6 (Mohs scale)
Color: Dark gray to black
Luster: Dull to vitreous

Identified October 25, 2025More igneous →

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Physical properties

Formation & geological history

Basalt forms from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the Earth's surface. It is a common volcanic rock and can be found in lava flows, shield volcanoes, oceanic crust, and dikes. Most basalts are Mesozoic to Cenozoic in age, but older basalts exist.

Uses & applications

Basalt is widely used in construction (e.g., aggregate for roads, concrete, asphalt), as paving stones, and in the production of basalt fibers for textiles and composites. Due to its durability and resistance to weathering, it is also used for building facades and monuments.

Geological facts

Basalt is the most common rock type in the Earth's crust, forming the bulk of the ocean floor. The giant's causeway in Northern Ireland and the Deccan Traps in India are famous examples of basaltic formations. It is also found on the Moon, Mars, and other terrestrial planets.

Field identification & locations

In the field, basalt can be identified by its dark color, fine-grained texture (though vesicles and phenocrysts may be present), and relative heaviness. It often has a rough, scoriaceous or vesicular texture on weathered surfaces due to gas bubbles. It is commonly found in volcanic regions, oceanic islands (like Hawaii), mid-ocean ridges, and continental flood basalt provinces. Collectors often seek out specimens with interesting textures or mineral 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