igneous
Dark gray rounded cobble, likely basalt or fine-grained metamorphic rock
Likely basalt (plagioclase–pyroxene rock); exact identification requires testing
AI-generated identification · may be incorrectFine-grained, massive, dark gray with mottled weathering and subvitreous-to-dull luster; hardness about 5–6 Mohs, no obvious cleavage, and typically specific gravity 2.8–3.1. The rounded shape indicates natural stream/beach abrasion or tumbling.
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Physical properties
Fine-grained, massive, dark gray with mottled weathering and subvitreous-to-dull luster; hardness about 5–6 Mohs, no obvious cleavage, and typically specific gravity 2.8–3.1. The rounded shape indicates natural stream/beach abrasion or tumbling.
Formation & geological history
If basalt, it formed from rapidly cooled mafic lava, commonly in volcanic fields or oceanic crust, from any age. Its present rounded form likely developed through prolonged transport and abrasion in a river, shore, or glacial deposit.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally dimension stone; generally low jewelry value, though attractive polished pieces may be collected.
Geological facts
Basalt commonly contains microscopic plagioclase and pyroxene crystals, making it appear uniformly fine-grained. Brown patches are likely iron-oxide weathering rather than metallic mineralization.
Field identification & locations
Look for a dense, dark, nonfoliated rock that may attract a magnet weakly if iron-rich; a streak, acid test, and fresh surface help distinguish it from slate, gabbro, or amphibolite. Common in volcanic and glacial regions worldwide.
