igneous
Weathered basalt cobble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically Mohs 5–6; dark gray to black, locally brownish from weathering; dull to subtly glassy luster, compact texture, and no obvious cleavage. The rounded shape and tan coating indicate stream or shoreline abrasion and surface weathering.
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Physical properties
Typically Mohs 5–6; dark gray to black, locally brownish from weathering; dull to subtly glassy luster, compact texture, and no obvious cleavage. The rounded shape and tan coating indicate stream or shoreline abrasion and surface weathering.
Formation & geological history
Formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the exact age and locality cannot be determined from the photograph.
Uses & applications
Used as aggregate, road stone, riprap, paving, and landscaping material; attractive polished pieces may be collected, but this specimen is not gem quality.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and ocean-floor crust.
Field identification & locations
Identify it by its heavy, fine-grained dark body, lack of visible large crystals, and possible weak magnetism from iron-bearing minerals. Common worldwide, especially volcanic regions and river gravels.
