igneous
Weathered river cobble, likely basaltic andesite
Fine-grained mafic volcanic rock; likely basalt or andesite, identification uncertain from image alone
AI-generated identification · may be incorrectDark gray-green, smooth and rounded from transport, with dull earthy luster and rusty brown weathering patches. Likely hardness 5–6 Mohs, massive with no visible cleavage and fine crystalline texture; composition probably plagioclase, pyroxene, and iron-bearing alteration minerals.
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Physical properties
Dark gray-green, smooth and rounded from transport, with dull earthy luster and rusty brown weathering patches. Likely hardness 5–6 Mohs, massive with no visible cleavage and fine crystalline texture; composition probably plagioclase, pyroxene, and iron-bearing alteration minerals.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, then was rounded by stream or shoreline abrasion and chemically weathered. Such volcanic rocks commonly range from Cenozoic to much older ages depending on locality.
Uses & applications
Commonly used as aggregate, road stone, riprap, landscaping stone, and occasionally a low-value lapidary material when polished.
Geological facts
The green-gray surface may reflect chlorite or other alteration, while orange-brown areas are likely iron oxides. A fresh broken surface would provide a more reliable identification than the weathered exterior.
Field identification & locations
Common in volcanic terrains and alluvial gravel beds; look for a dense, fine-grained, non-porous stone that may attract a weak magnet if iron-rich. Exact classification requires a fresh surface, streak, density, and ideally thin-section or laboratory analysis.
