metamorphic
Polished garnet-bearing gneiss
Biotite-quartz-feldspar gneiss with almandine garnet (garnet: Fe₃Al₂(SiO₄)₃)
AI-generated identification · may be incorrectCoarse, foliated texture with pale quartz/feldspar layers, dark biotite-rich bands, and a reddish-brown garnet crystal; garnet hardness 6.5–7.5 Mohs, vitreous luster, cubic crystal system, and no cleavage. The exposed surface appears sawn or polished, with possible resin or coating.
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Physical properties
Coarse, foliated texture with pale quartz/feldspar layers, dark biotite-rich bands, and a reddish-brown garnet crystal; garnet hardness 6.5–7.5 Mohs, vitreous luster, cubic crystal system, and no cleavage. The exposed surface appears sawn or polished, with possible resin or coating.
Formation & geological history
Formed by high-grade regional metamorphism of an impure sedimentary or igneous protolith, typically during Precambrian to Paleozoic mountain-building. Garnet grew during intense heat and pressure within the continental crust.
Uses & applications
Used as decorative facing stone, countertops, monuments, and geological specimen material; garnet-bearing pieces are mainly valued for appearance and education rather than gem use.
Geological facts
Gneiss commonly displays alternating compositional bands produced by mineral segregation during metamorphism. Garnet porphyroblasts can preserve inclusions that record changing pressure-temperature conditions.
Field identification & locations
Identify it by strong foliation, alternating light and dark bands, and hard red-brown garnet grains that resist scratching by steel. Common in ancient continental shields and metamorphic mountain belts, including Scandinavia, India, Madagascar, and the northeastern United States.
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