metamorphic
Gneiss
Banded Gneiss
Hardness: 6-7 (Mohs). Color: Alternating light (pink/white) and dark (grey/black) bands. Luster: Dull to pearly. Structure: Foliated with distinct compositional banding (gneissose banding). Cleavage: Poor. Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating light (pink/white) and dark (grey/black) bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs). Color: Alternating light (pink/white) and dark (grey/black) bands. Luster: Dull to pearly. Structure: Foliated with distinct compositional banding (gneissose banding). Cleavage: Poor. Specific Gravity: 2.6-2.9.
Formation & geological history
Formed under high-grade regional metamorphism where existing igneous (like granite) or sedimentary (like shale) rocks are subjected to extreme temperature and pressure, causing minerals to segregate into bands. Typically Precambrian to Paleozoic in age.
Uses & applications
Used as dimension stone for architecture, decorative garden stone, crushed stone for road construction, and occasionally for high-end countertops if the pattern is striking.
Geological facts
Gneiss represents one of the highest levels of metamorphism; if heated any further, it would begin to melt into magma and become a migmatite. It is often among the oldest rocks found on Earth's crust.
Field identification & locations
Identify by the alternating 'salty and pepper' bands of minerals like quartz/feldspar and biotite/hornblende. Commonly found in ancient shield areas like the Canadian Shield or the Scandinavian Shield. Field identification relies on the visible segregation of minerals into parallel layers.
