igneous
Fine-grained dark igneous rock, likely basalt
Basalt (approximate identification; commonly plagioclase, pyroxene, and minor olivine)
AI-generated identification · may be incorrectDark gray to black, fine-grained and massive, with a dull to slightly subvitreous luster; Mohs hardness about 5–6, no obvious cleavage, and typical specific gravity around 2.8–3.0. The image is not detailed enough to distinguish basalt confidently from fine-grained diabase or another mafic rock.
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Physical properties
Dark gray to black, fine-grained and massive, with a dull to slightly subvitreous luster; Mohs hardness about 5–6, no obvious cleavage, and typical specific gravity around 2.8–3.0. The image is not detailed enough to distinguish basalt confidently from fine-grained diabase or another mafic rock.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface; specimens may range from ancient volcanic flows to geologically recent lava. Basalt is common in oceanic crust and continental volcanic fields.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, building stone, and occasionally decorative material; unexceptional hand specimens have little commercial value.
Geological facts
Basalt commonly develops tiny vesicles, weathering rind, or scattered pale plagioclase crystals; none are clearly visible here.
Field identification & locations
In the field, identify its uniformly dark color, fine texture, heavy feel, and lack of visible layering; a streak test is usually pale gray, and magnetism may be weak. Common locations include Hawaii, Iceland, the Columbia River Basalt region, India, and oceanic seafloor.
