igneous
Rounded river cobble, likely basaltic volcanic rock
Fine-grained mafic igneous rock; likely basalt or andesite
AI-generated identification · may be incorrectApproximately Mohs 6–7; olive-brown to gray with subtle purple weathering, dull to weakly waxy luster, massive texture, and no visible cleavage. The smooth, rounded shape indicates prolonged abrasion, while the granular surface suggests fine crystalline material.
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Physical properties
Approximately Mohs 6–7; olive-brown to gray with subtle purple weathering, dull to weakly waxy luster, massive texture, and no visible cleavage. The smooth, rounded shape indicates prolonged abrasion, while the granular surface suggests fine crystalline material.
Formation & geological history
Likely formed from rapidly cooled lava during the Cenozoic or older, then weathered and transported in a river or glacial environment. Exact age and composition require a fresh surface, hardness test, and petrographic or chemical analysis.
Uses & applications
Commonly used as landscaping stone, riprap, paving aggregate, and decorative cobble; generally little gem value unless unusually patterned or collectible.
Geological facts
Basaltic cobbles commonly develop brown iron-oxide weathering rinds and may show greenish alteration minerals. Purple coloration can result from surface oxidation or minor mineral inclusions rather than true gemstone material.
Field identification & locations
Identify by its high hardness, fine-grained massive texture, lack of layering, and inability to scratch with a steel knife easily; basalt is common in volcanic terrains and river gravels worldwide. A streak, density, magnetism, and fresh fracture would improve identification.
