igneous
Dark gray basaltic river cobble
Basalt, likely fine-grained mafic volcanic rock (exact identification requires testing)
AI-generated identification · may be incorrectDark gray, dull to faintly subvitreous, fine-grained and massive, with rounded water-worn surfaces and minor fractures. Basalt commonly has Mohs hardness about 6, specific gravity 2.8–3.0, and no visible cleavage; tiny pale grains may be plagioclase or alteration minerals.
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Physical properties
Dark gray, dull to faintly subvitreous, fine-grained and massive, with rounded water-worn surfaces and minor fractures. Basalt commonly has Mohs hardness about 6, specific gravity 2.8–3.0, and no visible cleavage; tiny pale grains may be plagioclase or alteration minerals.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, producing a fine-grained texture; the specimen was subsequently rounded by stream or wave transport. Its precise geological age and source cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road stone, riprap, railroad ballast, and occasionally building or landscaping stone; ordinary cobbles have little gem or collector value.
Geological facts
Basalt is the most common volcanic rock and makes up much of oceanic crust. Its dark color reflects abundant pyroxene and iron-rich minerals, although weathering can produce lighter gray surfaces.
Field identification & locations
Check for a uniformly fine texture, high density, and lack of obvious quartz; a streak test is usually gray, and basalt generally does not fizz with dilute hydrochloric acid. Common in volcanic terrains and glacial or river gravels.
