igneous
Dark gray fine-grained stone, likely basalt
Basalt; mainly plagioclase feldspar, pyroxene, and sometimes olivine
AI-generated identification · may be incorrectHardness about 6–6.5 Mohs; dark gray to black, fine-grained, dull to slightly vitreous, with no visible cleavage. The pale streaks appear to be surface weathering, mineral veins, or lichen rather than defining crystals.
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Physical properties
Hardness about 6–6.5 Mohs; dark gray to black, fine-grained, dull to slightly vitreous, with no visible cleavage. The pale streaks appear to be surface weathering, mineral veins, or lichen rather than defining crystals.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, roadstone, riprap, railroad ballast, paving, and building stone; ordinary fragments have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. This specimen may also be a closely related dark fine-grained rock such as diabase, but texture and setting are insufficient for certainty.
Field identification & locations
Look for a dense, heavy, dark fine-grained surface and occasional tiny feldspar crystals; a streak test, fresh fracture, and hand lens would help distinguish basalt from concrete or other dark rocks. Common worldwide, especially volcanic regions.
