igneous
Rounded dark volcanic rock, likely basalt
Basalt; fine-grained plagioclase–pyroxene rock, commonly with olivine and magnetite
AI-generated identification · may be incorrectTypically Mohs 6–7, dark gray to black, dull to weakly vitreous, dense, and fine-grained; individual crystals are mostly microscopic, with no obvious cleavage. The smooth shape and surface indicate water or beach abrasion.
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Physical properties
Typically Mohs 6–7, dark gray to black, dull to weakly vitreous, dense, and fine-grained; individual crystals are mostly microscopic, with no obvious cleavage. The smooth shape and surface indicate water or beach abrasion.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces; specimens may range from recent to billions of years old. The image alone cannot establish a precise age or locality.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces are used decoratively but are generally not gemstone-grade.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. A magnet may show weak attraction if magnetite is present; vesicles, if present, indicate gas-rich lava, though none are obvious here.
Field identification & locations
Likely basalt based on its dark, compact, weather-resistant appearance, but similar gabbro, amphibolite, or diabase cannot be excluded visually. Common along volcanic coasts and river gravels; a streak, density, hardness, and thin-section test would improve identification.
