igneous
Mottled river cobble, likely porphyritic volcanic rock
Possibly porphyritic andesite or basalt; exact identification requires closer examination
AI-generated identification · may be incorrectDark gray, fine-grained matrix with conspicuous reddish and green patches that may be mineral grains or surface staining. Hardness and luster cannot be determined from the image; volcanic rocks commonly contain plagioclase and pyroxene, with Mohs hardness around 5–6.
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Physical properties
Dark gray, fine-grained matrix with conspicuous reddish and green patches that may be mineral grains or surface staining. Hardness and luster cannot be determined from the image; volcanic rocks commonly contain plagioclase and pyroxene, with Mohs hardness around 5–6.
Formation & geological history
Likely formed when magma cooled, with larger crystals developing before the remaining melt solidified; the rounded shape reflects transport and abrasion in water. Its age cannot be estimated visually.
Uses & applications
Typical uses for comparable volcanic stone include landscaping, decorative aggregate, and collectors’ specimens; this water-worn cobble has no evident gem value.
Geological facts
The red and green patches could be iron-oxide staining or distinct minerals, but the photograph alone does not establish their composition.
Field identification & locations
A rounded, hard cobble in a stream is consistent with river transport; a fresh broken surface, hand lens, and hardness tests would help distinguish volcanic rock from altered or stained alternatives. Volcanic cobbles occur in streams draining volcanic terrains worldwide.
