igneous
Weathered basalt or mafic volcanic rock
Basalt; typically plagioclase feldspar, clinopyroxene, and often olivine or iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to slightly earthy luster, with no visible large crystals; hardness about 5–6 Mohs, massive texture, poor or absent cleavage, and specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray-brown, fine-grained, dull to slightly earthy luster, with no visible large crystals; hardness about 5–6 Mohs, massive texture, poor or absent cleavage, and specific gravity roughly 2.8–3.0.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, generally during the Cenozoic or older volcanic episodes; the brown coloration likely reflects surface weathering and oxidation.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive dense pieces may be collected, though this specimen has limited gem value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust, including mid-ocean ridges and many volcanic islands.
Field identification & locations
Identify it by its heavy feel, dark fine-grained appearance, and lack of obvious layering; a streak, hardness, and acid test can help distinguish it from weathered sedimentary rocks. Common worldwide in volcanic regions and river gravels.
