igneous
Weathered basalt
Basalt, a mafic volcanic rock composed chiefly of plagioclase feldspar and pyroxene, with possible olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained and massive, with a dull to slightly vitreous luster; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The surface appears weathered, producing brownish-gray alteration tones.
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Physical properties
Dark gray to black, fine-grained and massive, with a dull to slightly vitreous luster; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The surface appears weathered, producing brownish-gray alteration tones.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. The specimen’s precise age and locality cannot be determined from the image.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, paving stone, and occasionally building or landscaping stone; generally not used as a gemstone.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively little quartz.
Field identification & locations
Identify it by its dark color, fine interlocking texture, heavy feel, and lack of visible quartz or large crystals; a streak, fresh break, and hand lens would improve confidence. Common localities include Hawaii, Iceland, the Deccan Traps, and oceanic volcanic regions.
