igneous
Dark igneous rock, possibly basalt or gabbro
Basalt–gabbroic rock; chiefly plagioclase feldspar and pyroxene, with possible olivine or iron oxides
AI-generated identification · may be incorrectBlack to dark gray, fine- to medium-grained, dull to slightly vitreous luster, with scattered brassy/yellow alteration or sulfide-looking patches. Typical hardness is about 5–6, with no obvious cleavage; exact identification requires a fresh surface and tests for grain size, magnetism, and density.
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Physical properties
Black to dark gray, fine- to medium-grained, dull to slightly vitreous luster, with scattered brassy/yellow alteration or sulfide-looking patches. Typical hardness is about 5–6, with no obvious cleavage; exact identification requires a fresh surface and tests for grain size, magnetism, and density.
Formation & geological history
Likely formed from mafic magma: basalt if fine-grained and volcanic, or gabbro if visibly coarse-grained and intrusive. Such rocks range from modern to Precambrian in age and commonly occur in oceanic crust, volcanic fields, dikes, and mafic plutons.
Uses & applications
Used as crushed aggregate, road stone, riprap, ballast, and occasionally as dark decorative stone; unattractive hand specimens generally have little commercial value.
Geological facts
The yellow-brown areas may represent weathered olivine, iron oxides, or sulfide minerals, but the photograph is insufficient to distinguish them.
Field identification & locations
Look for abundant dark minerals, pale plagioclase, high density, and possible weak magnetism; a glassy texture would favor basalt, while visibly interlocking coarse grains favor gabbro. Common sources include volcanic islands, oceanic crust, and mafic intrusions.
