metamorphic
Banded Gneiss
Banded Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light grey/white and dark grey/blue bands; Luster: Dull to slightly vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating light grey/white and dark grey/blue bands
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light grey/white and dark grey/blue bands; Luster: Dull to slightly vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor.
Formation & geological history
Formed from the high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures deep within the Earth's crust. Most specimens are Proterozoic or Archean in age.
Uses & applications
Commonly used as crushed stone in construction, road base, and occasionally as structural stone or decorative landscaping. High-quality slabs are sometimes used for countertops.
Geological facts
Gneiss represents one of the highest levels of metamorphism; its characteristic banding (gneissic foliation) occurs through a process called mineral segregation, where minerals separate into distinct layers.
Field identification & locations
Identified by 'ribbon-like' layering of alternating colors within a hard, crystalline matrix. Found in the cores of mountain ranges and ancient continental shields. Smooth, water-worn pebbles like this are common in riverbeds and glacial deposits.
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