igneous
Dark basalt cobble
Basalt; fine-grained mafic volcanic rock composed chiefly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectBlack to dark gray, fine-grained, massive, and weakly vitreous to dull; Mohs hardness about 6, with no visible cleavage and specific gravity roughly 2.8–3.0. The smooth rounded shape indicates water or beach abrasion, possibly with minor surface weathering.
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Physical properties
Black to dark gray, fine-grained, massive, and weakly vitreous to dull; Mohs hardness about 6, with no visible cleavage and specific gravity roughly 2.8–3.0. The smooth rounded shape indicates water or beach abrasion, possibly with minor surface weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly at oceanic ridges, hotspots, and continental volcanic fields. The specimen’s exact age and locality cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, paving stone, and occasionally architectural facing; rounded examples are mainly educational or decorative.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the oceanic crust. It commonly attracts a magnet weakly to moderately because of iron-bearing minerals.
Field identification & locations
Identify it by its uniformly dark color, fine texture, high density, and lack of large crystals; a streak, magnet, and fresh broken surface would help distinguish it from andesite or slag. Common worldwide, especially in volcanic regions and stream or beach gravels.
