igneous

Basalt

Basalt (Mafic volcanic rock)

Hardness: 6 on Mohs scale; Color: Dark grey to black, sometimes with a brownish-red weathered surface; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None, usually breaks with a conchoidal or irregular fracture; Specific gravity: 2.8–3.0.

Hardness: 6 on Mohs scale
Luster: Dull to sub-metallic

Identified May 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth, making up most of the ocean floor and large continental provinces.

Uses & applications

Extensively used in construction, specifically as an aggregate in concrete and asphalt, road base, and railroad ballast. Fine-grained varieties are also used for high-tensile basalt fiber and in the manufacturing of stone wool insulation.

Geological facts

Basalt is the primary rock type on the moon's lunar maria and also covers much of the surface of Mars and Venus. It often forms striking hexagonal columns during cooling, such as those seen at the Giant's Causeway in Ireland.

Field identification & locations

Identify in the field by its dark color, fine-grained texture (crystals too small to see with the naked eye), and high density. It is commonly found in volcanic regions, oceanic island arcs (like Hawaii), and mid-ocean ridges.

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