igneous
Polished river cobble, likely basalt or gabbro
Mafic igneous rock, probably basalt (plagioclase–pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray to black, fine- to medium-grained, weakly vitreous to dull luster with brown weathering bands; typical hardness about 5.5–6.5 Mohs, massive texture, no obvious cleavage, and density roughly 2.8–3.1 g/cm³.
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Physical properties
Dark gray to black, fine- to medium-grained, weakly vitreous to dull luster with brown weathering bands; typical hardness about 5.5–6.5 Mohs, massive texture, no obvious cleavage, and density roughly 2.8–3.1 g/cm³.
Formation & geological history
Likely formed when mafic lava cooled rapidly at or near Earth’s surface, then was rounded and smoothed by river transport and possibly polished. Its precise geological age and locality cannot be determined from the image.
Uses & applications
Used as landscaping stone, aggregate, riprap, pavers, and decorative polished stone; generally not a jewelry gemstone.
Geological facts
The elongated, glossy shape is consistent with prolonged abrasion in flowing water or artificial tumbling/polishing. Brown streaks may reflect iron oxidation or weathered olivine-rich material.
Field identification & locations
Identify with a hand lens by looking for tiny dark pyroxene grains and pale plagioclase; a streak test is usually inconclusive, while strong magnetism may suggest iron-rich basalt. Common worldwide in volcanic terrains and river gravels.
