igneous
Likely basalt or weathered dark volcanic rock
Basalt; fine-grained mafic rock composed mainly of plagioclase and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, dull to weakly vitreous, fine-grained, and massive; typical hardness about 5–6 Mohs, with no visible cleavage and specific gravity around 2.8–3.0. The rounded shape and surface wear suggest natural weathering or tumbling.
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Physical properties
Dark gray-brown, dull to weakly vitreous, fine-grained, and massive; typical hardness about 5–6 Mohs, with no visible cleavage and specific gravity around 2.8–3.0. The rounded shape and surface wear suggest natural weathering or tumbling.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Specimens may range from recent volcanic material to rocks hundreds of millions of years old; age cannot be determined from the image.
Uses & applications
Crushed basalt is widely used as aggregate, roadstone, railroad ballast, and concrete material; attractive varieties are used as landscaping stone and occasionally carvings. This small common specimen generally has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Vesicles, if present, may indicate gas bubbles trapped during lava cooling.
Field identification & locations
Identify it by its dark color, fine texture, high density, and inability to scratch easily with a steel knife; a streak test is usually gray. Common sources include Hawaii, Iceland, the Pacific Northwest, India, and oceanic islands.
