metamorphic
Dark mafic metamorphic rock, likely amphibolite
Amphibolite; chiefly hornblende amphibole and plagioclase feldspar
AI-generated identification · may be incorrectTypically dark gray to black, medium- to coarse-grained, with a dull to weakly vitreous luster and massive or faintly foliated texture. Hardness is approximately 5–6; individual grains may show amphibole cleavage, while the rock has no overall cleavage.
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Physical properties
Typically dark gray to black, medium- to coarse-grained, with a dull to weakly vitreous luster and massive or faintly foliated texture. Hardness is approximately 5–6; individual grains may show amphibole cleavage, while the rock has no overall cleavage.
Formation & geological history
Forms when basalt or gabbro undergoes medium- to high-grade regional metamorphism, commonly during mountain building; many examples are Precambrian to Paleozoic. The specimen’s weathered, rounded shape is consistent with stream or glacial transport.
Uses & applications
Used locally as crushed stone, aggregate, riprap, and dimension stone; attractive polished pieces may be sold as decorative stone, though it is not usually a jewelry gem.
Geological facts
Amphibolite commonly contains black hornblende with white-gray plagioclase, producing a salt-and-pepper appearance. It is denser and generally darker than many gneisses and schists.
Field identification & locations
Look for a heavy, dark rock with visible interlocking black amphibole and pale feldspar grains; a streak test is usually gray to white, and dilute acid produces no fizz. Common in metamorphic belts worldwide, including the Appalachians, Scandinavia, and the Canadian Shield.
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