metamorphic
Dark mafic metamorphic rock, likely amphibolite
Amphibolite; chiefly hornblende amphibole + plagioclase feldspar
AI-generated identification · may be incorrectTypically dark gray-green to black, medium- to coarse-grained, with a dull to weakly vitreous luster; hardness about 5–6, nonfoliated to weakly foliated, and no obvious cleavage at specimen scale. The image is also compatible with fine-grained gabbro or basalt, so exact identification requires a fresh surface and miner…
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Physical properties
Typically dark gray-green to black, medium- to coarse-grained, with a dull to weakly vitreous luster; hardness about 5–6, nonfoliated to weakly foliated, and no obvious cleavage at specimen scale. The image is also compatible with fine-grained gabbro or basalt, so exact identification requires a fresh surface and mineral testing.
Formation & geological history
Amphibolite forms when basaltic or gabbroic protoliths undergo medium- to high-grade metamorphism, commonly during mountain building; ages range from Precambrian to modern tectonic belts. It commonly occurs in metamorphic terranes and amphibolite-facies zones.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and occasionally dimension stone; attractive polished pieces may be used decoratively. Ordinary hand specimens have limited commercial value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich amphibole and feldspar. A magnet, streak test, acid test, and thin-section examination help distinguish it from basalt, gabbro, and hornfels.
Field identification & locations
Common in ancient continental shields and metamorphic belts worldwide, including Scandinavia, the Appalachians, Canada, and India. Look for elongated amphibole grains, green-black color, moderate heft, and a lack of vesicles; avoid calling it amphibolite without petrographic confirmation.
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