metamorphic
Dark gneiss or migmatitic amphibolite
Band ed gneiss; composition likely quartz–feldspar–biotite/amphibole, possibly migmatite
AI-generated identification · may be incorrectCoarse-grained, dark gray to black rock with irregular pale quartz-feldspar bands or veins; minerals have vitreous to subvitreous luster. Hardness is approximately 6–7 Mohs, with strong foliation but no obvious cleavage at specimen scale; exact identification requires thin-section or mineral testing.
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Physical properties
Coarse-grained, dark gray to black rock with irregular pale quartz-feldspar bands or veins; minerals have vitreous to subvitreous luster. Hardness is approximately 6–7 Mohs, with strong foliation but no obvious cleavage at specimen scale; exact identification requires thin-section or mineral testing.
Formation & geological history
Formed when pre-existing igneous or sedimentary rock underwent high-temperature, high-pressure metamorphism, commonly during Precambrian to Paleozoic mountain-building. Pale irregular zones may represent partial melting or felsic injections.
Uses & applications
Used as crushed stone, aggregate, riprap, and occasionally dimension stone; attractive polished pieces are collected or used decoratively rather than commonly made into jewelry.
Geological facts
The mottled light-and-dark pattern reflects metamorphic segregation of felsic minerals from dark mafic minerals. If the pale material forms true cross-cutting veins, the specimen may be migmatitic gneiss rather than ordinary banded gneiss.
Field identification & locations
Look for compositional banding, flattened or aligned minerals, and a granular texture; granite lacks strong mineral alignment, while amphibolite is typically darker and richer in amphibole. Common in ancient continental shields and metamorphic mountain belts worldwide.
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