igneous
Basalt
Mafic volcanic rock, chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to subtly glassy; hardness about 5–6, with no visible cleavage in the whole rock and specific gravity around 2.8–3.0. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
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Physical properties
Typically dark gray to black, fine-grained, and dull to subtly glassy; hardness about 5–6, with no visible cleavage in the whole rock and specific gravity around 2.8–3.0. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. Specimen age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railway ballast, and dimension stone; attractive polished pieces are sold as ornaments or teaching specimens.
Geological facts
Its fine texture reflects rapid cooling, preventing large crystals from developing. Vesicles may occur where trapped gas bubbles formed, though none are obvious here.
Field identification & locations
Identify it by its uniform dark color, fine grain, heavy feel, and lack of distinct large crystals; a streak test is usually gray-black. Common worldwide, especially near volcanic fields and beaches.
