igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic), often contains small crystals of plagioclase/olivine; Specific Gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark gray to black
Luster: Dull to sub-metallic

Identified March 4, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. It can be found in oceanic crust and volcanic regions; geological age varies from modern volcanic eruptions to billions of years old.

Uses & applications

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality specimens are used in stone masonry and as 'hot stones' for massage therapy.

Geological facts

Basalt is the most common rock on Earth's surface, making up most of the ocean floor. It is also found on the Moon and Mars, forming the dark plains known as 'maria'.

Field identification & locations

Identify in the field by its heavy weight (density), dark color, and fine-grained texture with no visible fossils. It is commonly found in volcanic areas like Hawaii, Iceland, or the Pacific Northwest. Collectors look for basalt with large 'phenocrysts' or vesicles filled with other minerals.

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