igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6 on Mohs scale; Color: dark grey to black (weathering to brown or rust); Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) sometimes containing larger phenocrysts; Cleavage: none; Specific gravity: 2.8 - 3.0.

Hardness: 6 on Mohs scale
Color: dark grey to black (weathering to brown or rust)
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 at or very near the Earth's surface. It can be found in oceanic crust, volcanic islands, and massive continental flood basalt provinces.

Uses & applications

Extensively used in construction for road base, concrete aggregate, and railroad ballast. Also used as dimension stone for tiles and landscaping or as 'hot stones' in spa treatments.

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 and Mars, forming the dark 'maria' (seas) visible from Earth.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture. It often weathers with a reddish-brown crust. Commonly found in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

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