igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock composed chiefly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray-brown, dense, dull to slightly granular luster, with tiny crystalline grains and no obvious cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. Its rounded shape indicates water or glacial abrasion.
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Physical properties
Dark gray-brown, dense, dull to slightly granular luster, with tiny crystalline grains and no obvious cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. Its rounded shape indicates water or glacial abrasion.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older volcanic rocks; the specimen was later weathered and transported as a cobble. Exact age and locality cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, and landscaping stone; attractive polished pieces may be sold as inexpensive decorative specimens.
Geological facts
Basalt is the most common volcanic rock on oceanic crust and forms much of the seafloor, mid-ocean ridges, and many continental lava flows.
Field identification & locations
Identify it by its dark color, fine interlocking texture, high density, and lack of visible layers or large crystals; a streak test is usually gray, and it commonly weakly attracts a magnet if iron-rich. Common in volcanic regions, river gravels, beaches, and glacial deposits.
