igneous
Likely weathered basalt or volcanic rock
Basalt; mainly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to weakly glassy surfaces with irregular fracture; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The green coloration may reflect weathering or alteration minerals such as chlorite, epidote, or serpentine.
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Physical properties
Dark gray-green, fine-grained, dull to weakly glassy surfaces with irregular fracture; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The green coloration may reflect weathering or alteration minerals such as chlorite, epidote, or serpentine.
Formation & geological history
Basalt forms when low-silica magma cools rapidly at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. Its exact geological age cannot be determined from the photograph; alteration likely occurred long after formation.
Uses & applications
Used as crushed aggregate, road stone, riprap, railroad ballast, and occasionally building stone. Ordinary pieces have little jewelry value, though attractive vesicular or altered specimens may be collected.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Its dark color and fine texture distinguish it from coarse-grained gabbro, while green alteration can make identification less certain.
Field identification & locations
Look for a dense, fine-grained dark rock, usually heavier than similarly sized sedimentary material; a streak test, hardness test, and inspection for tiny crystals or vesicles would help confirm it. Common settings include volcanic regions and former lava flows.
