igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray, fine-grained, dull to faintly subvitreous, with very low visible porosity and irregular fracture; typical hardness about 5–6 Mohs and specific gravity about 2.8–3.0. The rounded form and surface cracks indicate natural abrasion and weathering.
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Physical properties
Dark gray, fine-grained, dull to faintly subvitreous, with very low visible porosity and irregular fracture; typical hardness about 5–6 Mohs and specific gravity about 2.8–3.0. The rounded form and surface cracks indicate natural abrasion and weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, flood-basalt provinces, and volcanic fields. Its exact age cannot be determined from the image; basalt ranges from recent lava to rocks billions of years old.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and occasionally as paving or landscaping stone; this small cobble has mainly decorative or educational value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly forms the dark ocean floor. Its smooth shape likely resulted from river, beach, or glacial transport.
Field identification & locations
Identify it by its uniform dark color, fine texture, substantial weight, and lack of visible large crystals; a streak, magnetism test, and fresh broken surface would improve confidence. Common worldwide, especially near volcanic regions and basaltic shorelines.
