metamorphic
Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Tan, brown, and black banding; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture with distinct minerals like quartz and feldspar; Cleavage: Poor, splits along banding.
- Hardness
- 6-7 on Mohs scale
- Color
- Tan, brown, and black banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Tan, brown, and black banding; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture with distinct minerals like quartz and feldspar; Cleavage: Poor, splits along banding.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (such as granite or shale) subjected to high temperatures and pressures deep within the Earth's crust.
Uses & applications
Commonly used as crushed stone in construction, road base, and decorative landscaping. High-quality slabs can be used for countertops or building facades.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens in the Acasta Gneiss complex dating back over 4 billion years. The banding is caused by the segregation of light and dark minerals.
Field identification & locations
Identifiable in the field by its 'gneissose' banding—alternating layers of light (felsic) and dark (mafic) minerals. Often found in mountainous regions or glacial deposits like the river rock shown here.
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