igneous
Tumbled basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to brown, fine-grained, dull to faintly vitreous, with no visible crystals; hardness about 6 Mohs, dense, and lacking cleavage. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
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Physical properties
Typically dark gray to brown, fine-grained, dull to faintly vitreous, with no visible crystals; hardness about 6 Mohs, dense, and lacking cleavage. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. It later weathered and was transported as a stream or beach pebble.
Uses & applications
Used as aggregate, roadstone, riprap, ballast, landscaping stone, and decorative tumbled material; generally not used as a gemstone.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. The bluish-gray coloration may reflect weathering, surface coating, or minor minerals such as magnetite.
Field identification & locations
Identify by its dark, dense, fine-grained texture, lack of layering, and possible weak magnetism; distinguish it from slate by its massive texture and from glass by its granular surface. Common in volcanic regions, river gravels, and beaches.
