metamorphic
Granite Gneiss
Gneiss (High-grade metamorphic rock formed from granitic precursor)
Hardness: 6-7 on Mohs scale; Color: Salt and pepper appearance with pink, white, and black tones; Luster: Dull to vitreous; Structure: Foliated with distinct banding or streaks; Cleavage: Poor, usually fractures unevenly; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt and pepper appearance with pink, white, and black tones
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Salt and pepper appearance with pink, white, and black tones; Luster: Dull to vitreous; Structure: Foliated with distinct banding or streaks; Cleavage: Poor, usually fractures unevenly; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade metamorphism of granite or sedimentary rocks under intense pressure and temperature (regional metamorphism). Often dates back billions of years in continental shields.
Uses & applications
Used widely in construction as crushed stone, dimension stone for countertops and flooring, and for architectural monuments due to its durability.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada estimated to be over 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning to spark or glisten.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct layers of different colored minerals. It is ubiquitous in mountainous terrains and glacial till. Collectors value pieces with dramatic, swirled banding structures.
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