igneous
Weathered dark igneous rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectTypically 5–6 Mohs, dark gray to black, fine-grained, dull to weakly vitreous; crystals are usually microscopic, with no prominent cleavage. The rounded shape and mottled pale patches indicate weathering or surface encrustation.
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Physical properties
Typically 5–6 Mohs, dark gray to black, fine-grained, dull to weakly vitreous; crystals are usually microscopic, with no prominent cleavage. The rounded shape and mottled pale patches indicate weathering or surface encrustation.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust; its age cannot be determined from the photograph. The specimen is likely naturally rounded by stream, glacial, or soil abrasion.
Uses & applications
Used as crushed aggregate, roadstone, railroad ballast, riprap, and construction stone; attractive pieces may be collected or polished, though this specimen appears ordinary.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. It can be weakly magnetic when magnetite-rich.
Field identification & locations
Identify by its heavy feel, dark fine-grained texture, and inability to scratch easily with a steel knife; a streak, magnet test, and fresh broken surface would help confirm it. Common worldwide, especially volcanic regions and gravel deposits.
