igneous
Dark gray-brown stone, likely basalt or fine-grained volcanic rock
Basalt (tentative; likely plagioclase–pyroxene rock)
AI-generated identification · may be incorrectFine-grained, dull to weakly subvitreous, dark gray-brown and massive, with no visible cleavage or crystals; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The image alone cannot distinguish basalt from similar dark rocks such as andesite, hornfels, or weathered chert.
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Physical properties
Fine-grained, dull to weakly subvitreous, dark gray-brown and massive, with no visible cleavage or crystals; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The image alone cannot distinguish basalt from similar dark rocks such as andesite, hornfels, or weathered chert.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Most examples range from recent volcanic material to Precambrian age; this specimen’s age is indeterminate.
Uses & applications
Common basalt is used as crushed aggregate, road stone, railway ballast, riprap, and occasionally building stone; ordinary hand specimens have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and the main rock type of oceanic crust. Magnetic minerals may allow it to attract a strong magnet weakly, especially if iron-rich.
Field identification & locations
Look for a uniformly fine texture, dark color, heavy feel, and absence of obvious layering; a fresh surface is typically gray-black, while weathering turns brown. A streak, hardness, magnetism, and acid test would improve identification; common worldwide.
