metamorphic
Banded amphibolite (likely hornblende schist)
Amphibolite; chiefly hornblende + plagioclase, with possible biotite and garnet
AI-generated identification · may be incorrectDark greenish-black, coarse-grained, foliated and strongly banded, with a dull to subvitreous luster. Typical hardness is 5–6 Mohs; crystals are mostly interlocking amphibole and feldspar, with no obvious cleavage on the specimen surface.
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Physical properties
Dark greenish-black, coarse-grained, foliated and strongly banded, with a dull to subvitreous luster. Typical hardness is 5–6 Mohs; crystals are mostly interlocking amphibole and feldspar, with no obvious cleavage on the specimen surface.
Formation & geological history
Formed when basaltic or gabbroic rock underwent medium- to high-grade regional metamorphism, commonly during mountain building. Its exact geological age cannot be determined from the photograph and depends on the local bedrock.
Uses & applications
Used locally as crushed stone, riprap, aggregate, and occasionally dimension stone; attractive polished pieces are collectible but rarely gem-grade.
Geological facts
The dark mineral is commonly hornblende, whose two cleavage directions meet at about 60°/120°, while pale bands generally represent plagioclase-rich layers.
Field identification & locations
Look for a tough, heavy, dark rock with aligned amphibole crystals and alternating light-dark bands; it is commonly found in metamorphic belts worldwide, including the Appalachians, Canadian Shield, and Precambrian terrains.
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