igneous
Rounded basalt river cobbles
Basalt; fine-grained mafic volcanic rock, typically composed of plagioclase, pyroxene, and minor olivine
AI-generated identification · may be incorrectDark gray to black, dull to weakly silky weathered surfaces, fine-grained and massive; hardness about 6 Mohs, no obvious cleavage, specific gravity roughly 2.8–3.1. Their rounded shapes and smooth edges indicate prolonged abrasion, not crystal form.
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Physical properties
Dark gray to black, dull to weakly silky weathered surfaces, fine-grained and massive; hardness about 6 Mohs, no obvious cleavage, specific gravity roughly 2.8–3.1. Their rounded shapes and smooth edges indicate prolonged abrasion, not crystal form.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. These pieces were likely transported and rounded in a river or shoreline; their exact geological age cannot be determined from the image.
Uses & applications
Used as aggregate, road base, riprap, drainage stone, concrete aggregate, and landscaping material; attractive pieces may be used for garden or craft projects. They are generally not gemstone material.
Geological facts
Several specimens show vesicle-like pits or weathering cavities, while others appear dense and massive. Basalt is among the most abundant rocks on Earth and commonly forms dark lava flows and oceanic crust.
Field identification & locations
Identify by dark color, fine texture, high density, and lack of visible large crystals; a streak, magnetism test, and fresh broken surface would help distinguish basalt from andesite or dark sedimentary rock. Common in volcanic regions and gravel deposits.
