igneous
Rounded river cobble, likely basaltic rock
Fine-grained mafic volcanic rock, probably basalt or altered basalt
AI-generated identification · may be incorrectDark gray-brown, dull to weakly earthy luster, fine-grained and massive, with tan weathering veins or coatings. Estimated hardness 5–6 Mohs; cleavage absent and specific gravity roughly 2.8–3.1.
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Physical properties
Dark gray-brown, dull to weakly earthy luster, fine-grained and massive, with tan weathering veins or coatings. Estimated hardness 5–6 Mohs; cleavage absent and specific gravity roughly 2.8–3.1.
Formation & geological history
Likely formed from rapidly cooled mafic lava, then rounded by transport in a stream or along a beach; surface alteration produced the brown patches and veins. Basalt ranges from Precambrian to modern in age, but this specimen’s age cannot be determined visually.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, and occasionally dimension stone; this small cobble has mainly educational or decorative value.
Geological facts
Its smooth, oval shape reflects abrasion during transport rather than crystal growth. A magnet may show weak attraction if the rock contains magnetite.
Field identification & locations
Look for a dense, dark, fine-grained interior and possible tiny vesicles or iron-rich weathering; a fresh broken surface is more diagnostic than the weathered exterior. Common worldwide in volcanic terrains and river gravels.
