igneous
Likely basalt (possibly weathered andesite)
Fine-grained mafic volcanic rock, predominantly plagioclase and pyroxene; approximate composition equivalent to basalt
AI-generated identification · may be incorrectDark gray, dense, dull to weakly granular, with scattered pale mineral grains and rusty-brown weathering patches. Typical hardness is about 6 on Mohs scale; no obvious cleavage, and specific gravity is commonly 2.8–3.0.
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Physical properties
Dark gray, dense, dull to weakly granular, with scattered pale mineral grains and rusty-brown weathering patches. Typical hardness is about 6 on Mohs scale; no obvious cleavage, and specific gravity is commonly 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, producing a fine-grained texture; basalt ranges from Precambrian to modern in age and occurs in volcanic flows, oceanic crust, and flood-basalt provinces.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and occasionally building stone; attractive polished pieces may be collected but this specimen has limited gem value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. The pale specks are likely plagioclase, while brown areas reflect iron-bearing mineral weathering.
Field identification & locations
Common in volcanic regions, oceanic islands, and basalt plateaus worldwide. Identify it by its dark fine-grained matrix, high density, lack of visible large crystals, and inability to scratch it with a steel nail easily; definitive identification requires testing.
