metamorphic
Banded Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 Mohs; Color: Gray, white, and black banding; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Texture: Coarse-grained; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Gray, white, and black banding
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, white, and black banding; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Texture: Coarse-grained; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme pressure and temperature. This process leads to the segregation of light and dark minerals into layers. Dates range from Archean to recent mountain-building events.
Uses & applications
Used primarily as a building material, crushed stone for road construction, landscaping, and ornamental stone for countertops or gravestones due to its durability and attractive patterns.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' because the rock glitters.
Field identification & locations
Identify by looking for distinct parallel bands of different colors (foliation) where crystals are large enough to see with the naked eye. It often contains quartz, feldspar, and darker minerals like biotite or hornblende. Commonly found in the cores of mountain ranges and shield areas.
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