igneous
Polished basalt pebble
Basalt, a 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, opaque, and fine-grained with a dull to subtly vitreous luster; Mohs hardness about 5–6, no visible cleavage, and specific gravity approximately 2.8–3.0. The pale patches are likely weathering products, feldspar-rich areas, or attached mineral crust.
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Physical properties
Typically dark gray to black, opaque, and fine-grained with a dull to subtly vitreous luster; Mohs hardness about 5–6, no visible cleavage, and specific gravity approximately 2.8–3.0. The pale patches are likely weathering products, feldspar-rich areas, or attached mineral crust.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. The pebble was likely naturally water-rounded and subsequently polished; its precise geological age cannot be determined from the image.
Uses & applications
Used as aggregate, crushed stone, paving, landscaping material, and occasionally as a decorative or worry stone. Ordinary polished pieces have modest collecting value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron-bearing oxides.
Field identification & locations
Identify it by its uniformly dark, fine-grained, heavy appearance and lack of large crystals; a streak, fresh fracture, and magnet test can help distinguish it from black chert or slag. Common sources include volcanic regions and river or beach gravels worldwide.
