igneous
Rounded basalt cobble (probable)
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene
AI-generated identification · may be incorrectDark brown to gray, dull earthy luster, fine-grained and apparently massive; rounded by abrasion, with a pale weathering/crust patch. Typical hardness about 6, specific gravity 2.8–3.0, with no obvious cleavage or visible crystals.
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Physical properties
Dark brown to gray, dull earthy luster, fine-grained and apparently massive; rounded by abrasion, with a pale weathering/crust patch. Typical hardness about 6, specific gravity 2.8–3.0, with no obvious cleavage or visible crystals.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface. The cobble was likely transported and rounded by glacial meltwater or shoreline action; its exact age cannot be determined visually.
Uses & applications
Commonly used as aggregate, road stone, riprap, and landscaping material. This ordinary cobble has little jewelry or collector value.
Geological facts
The reported northeastern Minnesota location is consistent with glacially transported basalt or other dark volcanic/metamorphic cobbles from the Lake Superior region, though this image cannot establish a precise identification.
Field identification & locations
Look for a uniformly fine texture, dark color, substantial weight, and occasional tiny feldspar crystals; a streak, magnetism test, and fresh broken surface would help. It could alternatively be a weathered dark metamorphic rock such as amphibolite.
