metamorphic
Banded Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: alternating dark and light bands (grey, black, white, tan); Luster: dull to vitreous; Crystal structure: Coarse-grained foliated texture (gneissose banding).
- Hardness
- 6-7 on Mohs scale
- Color
- alternating dark and light bands (grey, black, white, tan)
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: alternating dark and light bands (grey, black, white, tan); Luster: dull to vitreous; Crystal structure: Coarse-grained foliated texture (gneissose banding).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. Gneiss is often billions of years old, dating back to the Precambrian era.
Uses & applications
Used in construction as crushed stone or dimension stone for countertops and building facades; small river-worn pebbles are popular for landscaping and aquarium decor.
Geological facts
Gneiss is among the oldest known rocks on Earth, including the Acasta Gneiss in Canada which is roughly 4 billion years old. The term comes from the German word 'gneist', meaning to spark.
Field identification & locations
Identified by its distinctive light and dark color bands resulting from mineral segregation. Commonly found in the roots of mountain ranges and deep within continental shields. Look for 'augen' (eye-shaped) mineral grains.
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