igneous
Rounded gray river cobble, likely basaltic andesite or weathered basalt
Aphanitic to fine-grained mafic volcanic rock; commonly plagioclase + pyroxene ± olivine, with possible alteration minerals
AI-generated identification · may be incorrectDark gray to gray-brown, dull to weakly waxy luster, fine-grained and massive, with no visible cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The smooth, oval shape results from stream abrasion and is not the original crystal form.
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Physical properties
Dark gray to gray-brown, dull to weakly waxy luster, fine-grained and massive, with no visible cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The smooth, oval shape results from stream abrasion and is not the original crystal form.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, likely during the Cenozoic or older depending on locality; subsequent river transport rounded the cobble. Exact identification and age require a fresh surface or laboratory testing.
Uses & applications
Used as landscaping stone, riprap, drainage aggregate, road base, and occasionally as a decorative garden or aquarium stone; generally not suitable as a gemstone.
Geological facts
The specimen appears to contain pale weathering patches and possibly small vesicles or inclusions, but the photograph cannot distinguish basalt from andesite, graywacke, or another volcanic cobble with certainty.
Field identification & locations
Look for a uniformly fine-grained, dense texture, dark color, and absence of foliation or obvious sedimentary layering; a streak, magnet test, hardness test, and fresh broken surface would help. Common in volcanic terrains and rivers draining them.
