igneous
Likely basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine and iron-titanium oxides
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, massive, and dull to subtly glassy; Mohs hardness about 5–6, no visible cleavage, and specific gravity about 2.8–3.1. The rounded, weathered surface may show vesicles or embedded brown alteration minerals.
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Physical properties
Typically dark gray to black, fine-grained, massive, and dull to subtly glassy; Mohs hardness about 5–6, no visible cleavage, and specific gravity about 2.8–3.1. The rounded, weathered surface may show vesicles or embedded brown alteration minerals.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact age cannot be determined from the image and may range from very young lava to billions of years old.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and dimension stone; attractive polished pieces are occasionally sold as decorative specimens.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron oxides.
Field identification & locations
Identify by its heavy feel, dark fine-grained texture, and inability to scratch easily with a steel tool; vinegar should not fizz unless carbonate veins are present. Common worldwide, especially near volcanic regions and stream gravels.
