igneous
Weathered mafic rock, likely basalt
Basalt (fine-grained aggregate dominated by plagioclase and pyroxene; identification is visual and uncertain)
AI-generated identification · may be incorrectDark gray-green, very fine-grained, dull to weakly granular luster, with rounded weathering and possible vesicles or alteration; hardness about 5–6 Mohs and no obvious cleavage. The smooth, flattened shape appears naturally water-worn or possibly polished.
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Physical properties
Dark gray-green, very fine-grained, dull to weakly granular luster, with rounded weathering and possible vesicles or alteration; hardness about 5–6 Mohs and no obvious cleavage. The smooth, flattened shape appears naturally water-worn or possibly polished.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust. It may range from recent to Precambrian in age; this specimen’s age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive pieces may be collected or polished, but this specimen has little gem value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly weathers to dark brown, red, or green surfaces as iron-bearing minerals alter.
Field identification & locations
Look for a dense dark color, fine texture, and occasional tiny feldspar crystals or vesicles; a streak, acid, density, and hand-lens examination would help distinguish basalt from dark metamorphic rocks or slag. Common worldwide, especially volcanic regions and river gravels.
