igneous
Weathered dark igneous rock, likely basalt
Basalt; commonly composed of plagioclase feldspar, clinopyroxene, and minor olivine or iron oxides
AI-generated identification · may be incorrectThe specimen appears dark gray-brown, fine-grained, dull to slightly vitreous, and irregularly rounded by weathering. Basalt typically has Mohs hardness about 6, no prominent cleavage as a rock, and specific gravity around 2.8–3.0; the image is insufficient to confirm mineral grains.
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Physical properties
The specimen appears dark gray-brown, fine-grained, dull to slightly vitreous, and irregularly rounded by weathering. Basalt typically has Mohs hardness about 6, no prominent cleavage as a rock, and specific gravity around 2.8–3.0; the image is insufficient to confirm mineral grains.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. Individual pieces may be millions to billions of years old, depending on locality.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, riprap, and sometimes building stone; ordinary pieces have little jewelry value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Vesicles, magnetic response, or visible olivine crystals can help distinguish varieties.
Field identification & locations
Look for a dense, dark fine-grained texture, possible tiny vesicles, and attraction to a strong magnet from iron minerals. Confirm with a fresh surface, streak, hardness test, and acid test; similar weathered rocks include andesite and slag.
