metamorphic
Granitic Gneiss
Gneiss (high-grade metamorphic rock)
Hardness: 6-7 Mohs; Color: Pink, white, gray, and black; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor, typically fractures unevenly; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Pink, white, gray, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Pink, white, gray, and black; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor, typically fractures unevenly; Specific Gravity: 2.6-2.9
Formation & geological history
Formed from the high-grade metamorphism of granite or sedimentary protoliths under intense heat and pressure. Typically associated with regional metamorphism during mountain-building events (orogenies).
Uses & applications
Used primarily as a construction material (crushed stone), decorative architectural stone, and occasionally for high-end countertops or landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is caused by the segregation of light minerals like quartz and feldspar from dark minerals like biotite and hornblende.
Field identification & locations
Identify in the field by looking for 'gneissose banding' where minerals are arranged in alternating light and dark layers. Common in continental shields and deep-seated metamorphic terrains.
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