igneous
Basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, massive, and dull to subtly glassy; hardness about 6, with no obvious cleavage and specific gravity roughly 2.8–3.0. The rounded shape indicates natural water abrasion or deliberate tumbling/cutting.
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Physical properties
Typically dark gray to black, fine-grained, massive, and dull to subtly glassy; hardness about 6, with no obvious cleavage and specific gravity roughly 2.8–3.0. The rounded shape indicates natural water abrasion or deliberate tumbling/cutting.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and lava flows. The specimen’s exact age and locality cannot be determined from the image.
Uses & applications
Used as aggregate, road stone, riprap, railway ballast, and building stone; attractive polished pieces are used decoratively. Common specimens generally have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals, while tiny crystals indicate rapid cooling.
Field identification & locations
Identify it by its uniformly dark, dense, fine-grained texture, lack of visible large crystals, and heavy weight; test against glass only cautiously because weathered basalt may scratch weakly. Common worldwide, especially around volcanic regions and riverbeds.
