igneous
Dark river-worn stone, likely basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene
AI-generated identification · may be incorrectGray-green to charcoal, dull to weakly subvitreous luster, massive and fine-grained with no visible crystals; Mohs hardness about 5–6 and typically no cleavage. The rounded shape indicates water abrasion.
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Physical properties
Gray-green to charcoal, dull to weakly subvitreous luster, massive and fine-grained with no visible crystals; Mohs hardness about 5–6 and typically no cleavage. The rounded shape indicates water abrasion.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, flood-basalt provinces, and volcanic islands; its age may range from recent to billions of years. The image alone cannot establish its exact age or locality.
Uses & applications
Used as crushed aggregate, road stone, riprap, paving, and occasionally decorative stone; ordinary pieces have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and commonly contains iron- and magnesium-rich minerals, giving it a dark color. Some varieties develop vesicles or green alteration minerals, but none are clearly visible here.
Field identification & locations
Confirm with a fresh surface, strong streak-free hardness, and lack of reaction to dilute acid; a magnet may show weak attraction. Commonly found in volcanic terrains and streambeds; exact identification could also include andesite or altered basalt.
