metamorphic
Banded Gneiss (River Cobble)
Gneiss (a high-grade metamorphic rock)
Hardness: 6-7 (depending on mineral composition); Color: Varies widely, often white, pink, grey, or black with distinct bands; Luster: Vitreous to dull, depending on constituent minerals; Crystal structure: Crystalline with an interlocking texture, often granular; Cleavage: Poor to indistinct; Specific gravity: 2.6-3.0
- Hardness
- 6-7 (depending on mineral composition)
- Luster
- Vitreous to dull, depending on constituent minerals
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Physical properties
Hardness: 6-7 (depending on mineral composition); Color: Varies widely, often white, pink, grey, or black with distinct bands; Luster: Vitreous to dull, depending on constituent minerals; Crystal structure: Crystalline with an interlocking texture, often granular; Cleavage: Poor to indistinct; Specific gravity: 2.6-3.0
Formation & geological history
Gneiss forms from the metamorphism of pre-existing igneous (like granite) or sedimentary (like shale) rocks under conditions of high temperature and pressure. The distinctive banding (gneissic banding) results from the segregation of different minerals into layers during recrystallization. Geological age can vary widely, from Precambrian to Cenozoic, depending on the specific parent rock and tectonic setting.
Uses & applications
Used as building stone, paving material, curbstone, and ornamental stone due to its attractive banding and durability. Also used as aggregate in construction.
Geological facts
Gneiss is one of the most common rocks in the Earth's continental crust and is found in many mountain ranges and ancient shield areas. The banding can be quite elaborate and beautiful, making it a popular choice for decorative applications. The rounded shape of this particular specimen indicates it has been transported and abraded, likely by water in a river or stream, forming a 'river cobble'.
Field identification & locations
Look for distinct banding of light and dark minerals, which is the most definitive characteristic of gneiss. It will often be hard and resistant to scratching. In the field, look for it in areas of high-grade metamorphism, such as in shields, ancient mountain belts, and riverbeds where it has been transported as cobbles or boulders. The rounded, smooth texture of this specific rock suggests it was found near a body of moving water.
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