igneous
Weathered basalt or mafic volcanic rock
Basalt; primarily plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to weakly vitreous, with irregular fracture and possible weathering rind; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.1. The image does not show diagnostic crystals clearly enough for a more specific identification.
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Physical properties
Dark gray to brown, fine-grained, dull to weakly vitreous, with irregular fracture and possible weathering rind; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.1. The image does not show diagnostic crystals clearly enough for a more specific identification.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic fields. It may range from geologically recent to billions of years old; the brown surface likely reflects oxidation and weathering.
Uses & applications
Commonly used as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive polished pieces may be sold as decorative specimens. This small, weathered piece has little commercial value without confirmed provenance.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Magnetic basalt may contain magnetite, allowing a strong response to a magnet.
Field identification & locations
Identify it by its dark, dense, fine-grained texture, lack of visible quartz, and possible magnetic response; a streak, hardness, density, and fresh broken surface would help confirm it. Common worldwide, especially volcanic regions and river gravels.
