metamorphic
Weathered mafic metamorphic rock, likely amphibolite or hornblende gneiss
Amphibolite; primarily hornblende amphibole and plagioclase feldspar, with possible biotite, quartz, and iron oxides
AI-generated identification · may be incorrectDark gray to brown, coarse crystalline, with a speckled/granular texture and dull to slightly vitreous luster; hardness about 5–6, typically massive to weakly foliated, with no obvious cleavage at specimen scale. Brown surfaces likely represent oxidation and weathering.
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Physical properties
Dark gray to brown, coarse crystalline, with a speckled/granular texture and dull to slightly vitreous luster; hardness about 5–6, typically massive to weakly foliated, with no obvious cleavage at specimen scale. Brown surfaces likely represent oxidation and weathering.
Formation & geological history
Formed when basalt or gabbro underwent medium- to high-grade regional metamorphism, commonly during Precambrian to Paleozoic mountain-building events. The rusty patches developed through later weathering of iron-bearing minerals.
Uses & applications
Used as crushed aggregate, dimension stone, riprap, and occasionally architectural stone; attractive polished pieces may be collected but are generally not a jewelry gemstone.
Geological facts
Amphibolite is identified by abundant dark hornblende surrounded by lighter plagioclase; it commonly preserves the chemical heritage of ancient volcanic rocks despite metamorphic recrystallization.
Field identification & locations
Common in metamorphic belts and exposed basement terrains worldwide, including Scandinavia, Canada, the northeastern United States, and the Appalachians. A definitive identification requires thin-section microscopy or mineral testing because the image could also show weathered gneiss or altered basalt.
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