igneous
Polished basalt cobble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to slightly vitreous; hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity approximately 2.8–3.0. The orange-brown patches are likely iron-oxide weathering or surface alteration.
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Physical properties
Typically dark gray to black, fine-grained, and dull to slightly vitreous; hardness about 5–6 Mohs, massive texture, no visible cleavage, and specific gravity approximately 2.8–3.0. The orange-brown patches are likely iron-oxide weathering or surface alteration.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. Most specimens are geologically young to Mesozoic or older, depending on locality.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, paving, and dimension stone; polished cobbles are decorative or collectible but generally low value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its fine texture reflects rapid cooling that prevented large crystals from forming.
Field identification & locations
Identify it by its heavy, uniformly fine-grained black appearance, magnetic response in some examples, and resistance to scratching by a steel knife. Common worldwide, especially near volcanic regions and stream gravels.
