igneous
Rounded gray river pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly basalt (plagioclase + pyroxene, sometimes olivine)
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly satin luster, with rounded water-worn surfaces and a lighter weathered fracture/patch. Typical hardness is about 5–6 Mohs; massive, non-cleavable, and relatively dense (approximately 2.7–3.1 g/cm³).
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Physical properties
Dark gray, fine-grained, dull to weakly satin luster, with rounded water-worn surfaces and a lighter weathered fracture/patch. Typical hardness is about 5–6 Mohs; massive, non-cleavable, and relatively dense (approximately 2.7–3.1 g/cm³).
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, then was rounded by stream or shoreline transport; its exact geological age and composition cannot be determined from the photograph.
Uses & applications
Useful mainly as a decorative pebble, landscaping stone, paperweight, or teaching specimen; generally low jewelry value unless unusually attractive or polished.
Geological facts
The smooth oval shape records abrasion during transport, while the pale band may be a weathered surface, mineral vein, or contact between pieces rather than a natural crystal layer.
Field identification & locations
Common in volcanic regions and glacial or river gravels. Field tests include a fresh gray-black fine texture, high density, no visible quartz grains, and resistance to a steel knife; vinegar reaction should be absent.
