igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray-green, dense, fine-grained, and dull to weakly lustered, with no visible crystal cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. Its smooth rounded form reflects water or abrasion rather than crystal shape.
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Physical properties
Dark gray-green, dense, fine-grained, and dull to weakly lustered, with no visible crystal cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. Its smooth rounded form reflects water or abrasion rather than crystal shape.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. The specimen is likely geologically old relative to its transport history, but its exact age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, paving stone, and occasionally architectural or decorative stone; ordinary cobbles have minimal jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Weathering may produce reddish-brown iron-oxide patches, like those visible here.
Field identification & locations
Identify it by its uniform dark color, fine texture, high density, and lack of obvious grains; a streak, magnetism test, and fresh fracture would improve confidence. Common in volcanic regions, riverbeds, beaches, and glacial deposits.
