igneous
Polished black basalt pebble
Basalt, a fine-grained mafic volcanic rock; primarily plagioclase, pyroxene, and possibly olivine
AI-generated identification · may be incorrectDark gray to black, dense, fine-grained, and commonly weakly magnetic; Mohs hardness about 5–6, dull to submetallic luster, no visible cleavage, and conchoidal-to-irregular fracture. The smooth surface indicates natural water wear or polishing; the small depression may be a vesicle or impact-like cavity.
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Physical properties
Dark gray to black, dense, fine-grained, and commonly weakly magnetic; Mohs hardness about 5–6, dull to submetallic luster, no visible cleavage, and conchoidal-to-irregular fracture. The smooth surface indicates natural water wear or polishing; the small depression may be a vesicle or impact-like cavity.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and lava flows. Most examples range from recent to several billion years old, depending on source.
Uses & applications
Used as aggregate, roadstone, building stone, riprap, and occasionally for carvings, worry stones, and ornamental polished pieces.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and relatively low silica content. A magnet test may show weak attraction if magnetite is present.
Field identification & locations
Identify it by its black, heavy, fine-grained appearance and lack of distinct crystals; it should not fizz with dilute acid. Common sources include volcanic regions, river gravels, beaches, and landscaping stone.
