metamorphic
Garnet-bearing metamorphic rock, likely garnet gneiss or garnet amphibolite
Garnet-bearing gneiss; red crystals likely almandine garnet, Fe₃Al₂(SiO₄)₃
AI-generated identification · may be incorrectLight gray, granular matrix with scattered reddish-brown garnet crystals and dark amphibole/biotite-rich patches; garnet hardness 6.5–7.5 Mohs, vitreous luster, cubic crystal structure, and no cleavage. The host rock is coarse-grained and appears weakly foliated.
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Physical properties
Light gray, granular matrix with scattered reddish-brown garnet crystals and dark amphibole/biotite-rich patches; garnet hardness 6.5–7.5 Mohs, vitreous luster, cubic crystal structure, and no cleavage. The host rock is coarse-grained and appears weakly foliated.
Formation & geological history
Formed when aluminous sedimentary or igneous rock underwent medium- to high-grade metamorphism during mountain building, commonly hundreds of millions to billions of years ago. Garnet crystallized during elevated temperature and pressure.
Uses & applications
Used mainly as a decorative and educational specimen; crushed garnet is an industrial abrasive and water-jet cutting medium, while sound gneiss may be used for building stone and aggregate.
Geological facts
The red grains are probably almandine garnet, though exact identification requires testing. Garnet commonly grows as porphyroblasts, producing conspicuous crystals larger than the surrounding matrix.
Field identification & locations
Identify by the hard, glassy red grains that resist scratching and lack cleavage, contrasting with the foliated quartz-feldspar-rich host. Common in metamorphic belts worldwide, including New England, the Adirondacks, Scandinavia, and the Himalaya.
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