igneous
Weathered basaltic cobble
Basalt, likely vesicular or altered basalt; chiefly plagioclase and pyroxene with possible olivine and iron-oxide staining
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to subtly glassy, with rusty orange weathering; likely Mohs 5–6, massive with no obvious cleavage, and specific gravity about 2.8–3.1. The rounded shape indicates water or sediment transport and weathering.
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Physical properties
Dark gray to brown, fine-grained, dull to subtly glassy, with rusty orange weathering; likely Mohs 5–6, massive with no obvious cleavage, and specific gravity about 2.8–3.1. The rounded shape indicates water or sediment transport and weathering.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the image; the orange-brown surface is likely oxidation of iron-bearing minerals.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, and occasionally dimension stone or landscaping material; generally not gem-quality. A typical cobble has negligible commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly contains microscopic crystals that require a hand lens to distinguish. Brown coloration can result from weathering to iron oxides or alteration minerals.
Field identification & locations
Identify it by its heavy, hard, fine-grained texture, dark mafic color, and lack of visible quartz or large crystals; a streak, hardness, and fresh broken surface would improve confidence. Common sources include stream gravels, volcanic terrains, and glacial deposits.
