Igneous

Basalt

Vesicular Basalt (Mafic volcanic rock)

Hardness: 5-6 Mohs; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8 - 3.0. Note the 'vesicles' (small holes) and amygdales (mineral-filled holes) visible on the surface.

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

Identified March 5, 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. The pits seen in the image are caused by gas bubbles being trapped as the lava solidified. Basalt ranges from recent volcanic activity to billions of years old.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Dense varieties are used for floor tiles and monument stone. It is also used in the manufacture of rock 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, Mars, and Venus. Famous basaltic formations include the Giant's Causeway in Northern Ireland.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often exhibits columnar jointing in large outcrops. Common in volcanic regions such as the Pacific Northwest of the US, Iceland, and Hawaii.

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