igneous
Likely weathered basalt or fine-grained volcanic rock
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectSmall rounded gray-brown cobble with a dull, granular surface and numerous tiny pits or weathering cavities; likely Mohs 5–6, opaque, massive, and lacking visible cleavage. Exact identification is limited by the photograph.
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Physical properties
Small rounded gray-brown cobble with a dull, granular surface and numerous tiny pits or weathering cavities; likely Mohs 5–6, opaque, massive, and lacking visible cleavage. Exact identification is limited by the photograph.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic flows; specimens may range from geologically young to hundreds of millions of years old. The rounded shape indicates stream, beach, or other abrasion.
Uses & applications
Commonly used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces may be collected but are usually low-value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly contains microscopic crystals that require magnification to see.
Field identification & locations
Identify it by its dark gray color, fine grain, high density, and lack of obvious layered structure; a streak, hardness, magnetism, and fresh broken surface would help distinguish basalt from weathered sandstone or slag. Common worldwide, especially volcanic regions and river gravels.
