igneous
Rounded basalt cobble (probable)
Basalt; fine-grained mafic volcanic rock, primarily plagioclase and pyroxene
AI-generated identification · may be incorrectDark gray, dull to faintly silky, very fine-grained, and strongly rounded by abrasion; typical hardness is about 6 Mohs, with no visible cleavage. The straight surface line is likely a natural fracture or vein, while the small dark spot may be a mafic mineral or pit.
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Physical properties
Dark gray, dull to faintly silky, very fine-grained, and strongly rounded by abrasion; typical hardness is about 6 Mohs, with no visible cleavage. The straight surface line is likely a natural fracture or vein, while the small dark spot may be a mafic mineral or pit.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact age and locality cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally building stone; unremarkable cobbles have little jewelry or collector use.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly contains microscopic crystals, so individual minerals are difficult to see without magnification.
Field identification & locations
Test for a fine-grained texture, gray-black streak or powder, and density around 2.8–3.0 g/cm³; it should not fizz with dilute hydrochloric acid. Common along volcanic terrains and river or beach gravels.
