igneous
Black basalt pebble
Basalt; fine-grained mafic volcanic rock, commonly composed of plagioclase, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with dull to weakly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity around 2.8–3.0. The rounded, worn shape suggests water or beach abrasion; the image does not resolve exact mineral texture.
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Physical properties
Typically dark gray to black, fine-grained, with dull to weakly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity around 2.8–3.0. The rounded, worn shape suggests water or beach abrasion; the image does not resolve exact mineral texture.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly from oceanic volcanism, lava flows, or hotspots. It occurs from ancient Precambrian terrains to modern eruptions; this specimen’s age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, concrete material, and dimension stone; attractive polished pieces are occasionally sold as decorative specimens. It is generally not a gemstone.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the oceanic crust. Magnetic basalt may contain magnetite and can show weak attraction to a strong magnet.
Field identification & locations
Commonly found in volcanic regions, river gravels, and beaches; identify by its heavy black fine-grained appearance, lack of visible large crystals, and resistance to scratching. A streak, magnet, fresh break, and acid test would improve confidence.
