igneous
Rounded gray river cobble
Likely basalt or fine-grained mafic volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly subvitreous luster, and smoothly water-worn with no visible crystals or cleavage. Likely hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1.
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Physical properties
Dark gray, fine-grained, dull to weakly subvitreous luster, and smoothly water-worn with no visible crystals or cleavage. Likely hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1.
Formation & geological history
Probably formed as an extrusive volcanic rock when mafic lava cooled rapidly, then was transported and rounded by running water or wave action. Its geological age cannot be determined from the photograph.
Uses & applications
Commonly used as landscaping stone, decorative cobble, aggregate, and occasionally in lapidary work; this specimen has little commercial value unless unusually attractive or locality-proven.
Geological facts
Basalt commonly contains plagioclase and pyroxene, sometimes olivine or magnetite, producing its dark color and relatively high density.
Field identification & locations
Look for a uniformly fine texture, dark gray-black color, substantial weight, and possible magnetic response; a streak, acid, density, and thin-section test can distinguish it from andesite, slate, or metamorphic rocks. Common in volcanic regions and river deposits worldwide.
