igneous
Dark mafic igneous rock, likely basalt or gabbro
Basaltic rock composed chiefly of plagioclase feldspar and pyroxene, with possible olivine and iron-oxide minerals
AI-generated identification · may be incorrectDark gray, fine- to medium-grained, massive, and weathered, with pale feldspar-rich patches or veins; typically hardness 5–6, dull to weakly subvitreous luster, negligible cleavage as a whole, and specific gravity about 2.8–3.1.
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Physical properties
Dark gray, fine- to medium-grained, massive, and weathered, with pale feldspar-rich patches or veins; typically hardness 5–6, dull to weakly subvitreous luster, negligible cleavage as a whole, and specific gravity about 2.8–3.1.
Formation & geological history
Formed from cooling mafic magma, possibly as an extrusive basalt or a coarse intrusive equivalent; generally Mesozoic to Precambrian depending on locality, but the image alone cannot establish age.
Uses & applications
Used as crushed aggregate, road stone, riprap, railroad ballast, and occasionally building or decorative stone; ordinary hand specimens have little commercial value.
Geological facts
The pale material may be feldspar, weathered alteration minerals, or quartz-rich veining. A fresh broken surface and grain size help distinguish basalt from gabbro.
Field identification & locations
Look for abundant dark pyroxene and pale plagioclase, high density, and resistance to scratching with a steel knife; a streak test is usually unhelpful. Common in volcanic regions, oceanic crust, dikes, and mafic intrusions.
