igneous
Dark mafic rock, likely basalt or amphibolite
Probable basalt (plagioclase feldspar + pyroxene ± olivine); definitive identification requires a fresh surface or thin section
AI-generated identification · may be incorrectDark gray-green, fine- to medium-grained, dull to weakly vitreous; likely hardness 5–6, massive texture, no obvious cleavage, and specific gravity around 2.8–3.2. The weathered brown coating may reflect iron oxidation.
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Physical properties
Dark gray-green, fine- to medium-grained, dull to weakly vitreous; likely hardness 5–6, massive texture, no obvious cleavage, and specific gravity around 2.8–3.2. The weathered brown coating may reflect iron oxidation.
Formation & geological history
If basalt, it formed from rapidly cooled mafic lava, commonly in volcanic settings and oceanic crust, generally at any age. The specimen’s apparent grain size also permits weathered amphibolite, a metamorphosed basalt; field context is needed.
Uses & applications
Basalt is widely used as crushed aggregate, road stone, riprap, and dimension stone; attractive pieces may be collected, but this specimen appears ordinary.
Geological facts
Basalt commonly contains calcium-rich plagioclase and pyroxene, while amphibolite is dominated by hornblende and plagioclase. A streak, magnetism test, and fresh break can help distinguish alternatives.
Field identification & locations
Common in volcanic and metamorphic terrains, including parts of North Carolina. Look for dense dark color, small interlocking crystals, and possible weak magnetism; acid reaction should be absent unless carbonate-filled fractures occur.
