igneous
Rounded basalt cobble
Basalt (fine-grained mafic volcanic rock)
AI-generated identification · may be incorrectTypically dark gray to brown, fine-grained, and dull to slightly vitreous; hardness about 5–6 Mohs, with no visible cleavage and a dense texture. The surface appears weathered and smoothly rounded, likely by water or abrasion.
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Physical properties
Typically dark gray to brown, fine-grained, and dull to slightly vitreous; hardness about 5–6 Mohs, with no visible cleavage and a dense texture. The surface appears weathered and smoothly rounded, likely by water or abrasion.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, paving stone, and occasionally decorative landscaping material; an ordinary cobble has little jewelry value.
Geological facts
The rusty-brown color likely reflects oxidation of iron-bearing minerals during weathering. Fine grain and strong density distinguish basalt from similarly dark, coarse-grained gabbro.
Field identification & locations
Look for a heavy, compact rock with tiny or invisible crystals, dark coloration, and occasional small vesicles or plagioclase specks; a streak, magnetism, and fresh broken surface can help confirm it. Common in volcanic regions and streambeds downstream from basalt outcrops.
