igneous
Basalt cobble
Basalt, a fine-grained mafic volcanic rock composed chiefly of plagioclase feldspar and pyroxene, commonly with minor olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray-green, rounded, and dull to faintly satin-lustrous; typically very fine-grained with no visible crystals. Hardness is about 6 Mohs, with no prominent cleavage and specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray-green, rounded, and dull to faintly satin-lustrous; typically very fine-grained with no visible crystals. Hardness is about 6 Mohs, with no prominent cleavage and specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Basalt ranges from ancient Archean rocks to modern eruptions.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally dimension stone; attractive polished pieces may be sold as decor or specimens.
Geological facts
The smooth, oval shape indicates abrasion by a river, beach, or glacial environment rather than its original volcanic form.
Field identification & locations
Identify it by its dark color, fine texture, high density, and lack of obvious sedimentary layering; a streak test is usually gray. Common worldwide, especially in volcanic regions, river gravels, and beaches.
