igneous
Dark gray stream-worn cobble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly basalt (plagioclase ± pyroxene ± olivine)
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dark gray-brown, dull to faintly subvitreous, massive and fine-grained, with no visible cleavage. Rounded shape indicates abrasion by water or glacial transport; exact identification requires a fresh surface or testing.
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Physical properties
Hardness about 5.5–6.5 Mohs; dark gray-brown, dull to faintly subvitreous, massive and fine-grained, with no visible cleavage. Rounded shape indicates abrasion by water or glacial transport; exact identification requires a fresh surface or testing.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, generally from the Cenozoic or older volcanic activity, then became rounded in a stream, beach, or glacial deposit.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally decorative material; this ordinary cobble has little jewelry value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly forms oceanic crust and extensive lava flows.
Field identification & locations
Look for a dense, fine-grained dark interior, lack of foliation, and occasional tiny feldspar or olivine grains; a streak test, magnetism check, and fresh break help distinguish basalt from andesite or dark metamorphic rocks. Common worldwide in volcanic and glacial regions.
