igneous
Dark gray rounded river stone, likely basalt or fine-grained metamorphic rock
Probably basalt (plagioclase–pyroxene rock); exact identification is uncertain from the image
AI-generated identification · may be incorrectDark gray, fine-grained, dull to slightly satin luster, massive and strongly rounded by abrasion; basalt typically has Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0.
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Physical properties
Dark gray, fine-grained, dull to slightly satin luster, massive and strongly rounded by abrasion; basalt typically has Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0.
Formation & geological history
If basalt, it formed from rapidly cooled mafic lava, commonly in volcanic flows, then was rounded by water or glacial transport. Basalt ranges from Precambrian to modern in age; a photograph cannot establish its age or exact composition.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, and building stone; attractive polished pieces may be used decoratively or collected, but this specimen appears low-value.
Geological facts
Its uniform dark color and fine texture are consistent with basalt, though slate, amphibolite, or other dark rocks could look similar. A fresh broken surface and a streak, magnetism, or thin-section analysis would improve identification.
Field identification & locations
Likely found in volcanic or glacial regions and stream gravels. Look for tiny crystals, vesicles, or weak magnetism; avoid relying on color alone because dark metamorphic rocks can be similar.
