metamorphic
Garnet Gneiss
Gneiss containing almandine-pyrope garnet
Hardness: 6-7.5 (depending on mineral ratio); Color: pinkish-red, grey, and black; Luster: Vitreous to dull; Crystal Structure: Foliated pattern; Cleavage: None (in garnet) to good (in mica); Specific Gravity: 2.7–3.2
- Hardness
- 6-7
- Color
- pinkish-red, grey, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7.5 (depending on mineral ratio); Color: pinkish-red, grey, and black; Luster: Vitreous to dull; Crystal Structure: Foliated pattern; Cleavage: None (in garnet) to good (in mica); Specific Gravity: 2.7–3.2
Formation & geological history
Formed through high-grade regional metamorphism of clay-rich sedimentary rocks or igneous precursors. Under high pressure and temperature, minerals recrystallize into distinct bands and large garnet crystals (porphyroblasts).
Uses & applications
Used as dimension stone for architectural facades, countertops, and flooring. Lower quality specimens are crushed for industrial abrasives or water filtration media.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens dating back over 4 billion years. Large garnet 'eyes' in gneiss are called porphyroblasts.
Field identification & locations
Identify by looking for distinct mineral banding (gneissose banding) and hard, rounded dark-red crystals embedded in a crystalline matrix. Found in mountainous or shield regions like the Adirondacks, Scandinavia, and India.
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