metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands of black, grey, white, or tan; Luster: Dull to vitreous; Structure: Foliated with distinct 'gneissic banding'; Cleavage: Poor, typically breaks unevenly across bands.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands of black, grey, white, or tan; Luster: Dull to vitreous; Structure: Foliated with distinct 'gneissic banding'; Cleavage: Poor, typically breaks unevenly across bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. This process causes minerals to recrystallize and segregate into layers. Many gneisses are from the Precambrian era, billions of years old.
Uses & applications
Used primarily as decorative crushed stone, construction aggregate, and dimension stone for countertops or flooring when the banding is aesthetically pleasing.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years ago. The distinct banding is a result of chemical migration during intense pressure.
Field identification & locations
Identify by looking for 'gneissic flowering'—parallel bands of minerals where the light bands are quartz/feldspar and dark bands are biotite/amphibole. Commonly found in mountainous regions or stable continental interiors (cratons).
More like this
