igneous
Dark mafic rock, likely basalt
Basalt (fine-grained assemblage chiefly plagioclase and pyroxene, commonly with olivine and iron oxides)
AI-generated identification · may be incorrectDark gray to black, fine-grained, massive, and mostly dull to subvitreous; likely Mohs hardness 5–6, with no obvious cleavage and density about 2.8–3.1 g/cm³. The image suggests a weathered, vesicle-poor fragment, but mineral identification is limited without a close-up or tests.
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Physical properties
Dark gray to black, fine-grained, massive, and mostly dull to subvitreous; likely Mohs hardness 5–6, with no obvious cleavage and density about 2.8–3.1 g/cm³. The image suggests a weathered, vesicle-poor fragment, but mineral identification is limited without a close-up or tests.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and hotspot settings. Its age may range from recent eruptions to Precambrian examples.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished basalt is occasionally used decoratively. Ordinary hand specimens generally have little market value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It may be magnetic because of magnetite and can develop reddish-brown weathering crusts.
Field identification & locations
In the field, look for uniform dark color, very fine texture, heavy weight, and possible tiny crystals or vesicles; a streak test is usually unhelpful. A hand lens, magnet, hardness test, and thin section would distinguish it from andesite, slag, or dark metamorphic rocks; common locations include volcanic regions worldwide.
