igneous
Dark mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly vitreous luster; hardness about 5–6, no obvious cleavage, and typically dense (specific gravity ~2.8–3.0). The image suggests a weathered, vesicle-poor fragment, though exact identification requires testing.
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Physical properties
Fine-grained, dark gray to black, dull to weakly vitreous luster; hardness about 5–6, no obvious cleavage, and typically dense (specific gravity ~2.8–3.0). The image suggests a weathered, vesicle-poor fragment, though exact identification requires testing.
Formation & geological history
An extrusive igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Basalt ranges from recent volcanic material to rocks hundreds of millions or billions of years old.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, building stone, and basalt fiber; attractive polished pieces may be collected but usually have low gem value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of the ocean floor. Magnetic basalt may contain magnetite and can weakly attract a magnet.
Field identification & locations
Look for a uniformly dark, heavy, fine-grained texture; a streak is usually gray, and a fresh break is dark and crystalline. Common worldwide, especially Hawaii, Iceland, oceanic islands, and volcanic provinces.
