igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray-brown, rounded and weathered, with a dull earthy-to-subtle vitreous luster and no visible crystals. Hardness is approximately 5–6 Mohs; massive, dense, and lacking cleavage, with typical specific gravity around 2.8–3.0.
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Physical properties
Dark gray-brown, rounded and weathered, with a dull earthy-to-subtle vitreous luster and no visible crystals. Hardness is approximately 5–6 Mohs; massive, dense, and lacking cleavage, with typical specific gravity around 2.8–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Its age cannot be determined from the image; basalt ranges from ancient to geologically modern.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a carving or decorative material; this weathered cobble has little gem value.
Geological facts
The rounded shape indicates transport and abrasion, likely in a stream, beach, or landscaping gravel bed. Brown coloration is probably surface oxidation and weathering of iron-bearing minerals.
Field identification & locations
Identify by its fine texture, heavy feel, dark mafic color, and inability to scratch easily with a steel knife; it typically does not fizz with dilute acid. Common worldwide, especially volcanic regions and gravel deposits.
