igneous
Dark fine-grained igneous rock, likely basalt
Basalt; commonly composed of plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectTypically black to dark gray, fine-grained, and dull to slightly vitreous; Mohs hardness about 5–6, with massive texture and no obvious cleavage. The specimen appears weathered and may contain small vesicles or phenocrysts, though image-based identification is limited.
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Physical properties
Typically black to dark gray, fine-grained, and dull to slightly vitreous; Mohs hardness about 5–6, with massive texture and no obvious cleavage. The specimen appears weathered and may contain small vesicles or phenocrysts, though image-based identification is limited.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and continental flood-basalt provinces. Its age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, and building stone; attractive polished basalt is used decoratively. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly weathers to dark brown or reddish soils rich in iron and magnesium.
Field identification & locations
Identify it by its dense dark color, fine grain, and hardness; a streak, magnetism test, and hand lens can help distinguish it from shale or metamorphic rocks. Common worldwide, especially volcanic regions and river gravels.
