igneous
Weathered mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, pyroxene, and minor olivine or iron oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained and mostly dull, with small pale crystals and rusty weathering spots; hardness about 5–6, generally massive with no visible cleavage. The rounded shape reflects water or abrasion rather than crystal form.
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Physical properties
Dark gray to black, fine-grained and mostly dull, with small pale crystals and rusty weathering spots; hardness about 5–6, generally massive with no visible cleavage. The rounded shape reflects water or abrasion rather than crystal form.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. Exact age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, riprap, and occasionally building stone; ordinary specimens have little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. The specimen may also be a related dark volcanic rock such as andesite, but texture and color favor basalt.
Field identification & locations
Identify by its dark color, fine texture, high density, and lack of visible sedimentary layering; a streak, hardness, and fresh broken surface would help distinguish it from slag or altered rock. Common worldwide, especially volcanic regions and riverbeds.
