igneous
Dark basalt fragment
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, generally dull to subtly vitreous; hardness about 6 Mohs, massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0. The irregular broken faces and faint speckling are consistent with basalt, though lighting limits certainty.
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Physical properties
Fine-grained, dark gray to black, generally dull to subtly vitreous; hardness about 6 Mohs, massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0. The irregular broken faces and faint speckling are consistent with basalt, though lighting limits certainty.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions. Basalt ranges from ancient Precambrian flows to modern volcanic deposits.
Uses & applications
Crushed basalt is used for aggregate, road base, railway ballast, concrete, and stone wool; attractive varieties are used as landscaping stone and occasionally polished décor.
Geological facts
Basalt is the most widespread volcanic rock and forms much of the oceanic crust, including seafloor around mid-ocean ridges.
Field identification & locations
Identify it by its heavy feel, dark color, fine grain, and lack of obvious layers or large crystals; it commonly occurs in lava flows, volcanic fields, and oceanic crust. A streak, magnet test, and fresh fracture would improve identification.
