metamorphic
Gneiss with Quartz Veining
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding or layering; Cleavage: Poor, typically breaks along planes of mineral alignment; Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding or layering; Cleavage: Poor, typically breaks along planes of mineral alignment; Specific Gravity: 2.6-3.0.
Formation & geological history
Formed through high-pressure, high-temperature metamorphism of granite or sedimentary protoliths. The banding occurs as minerals like quartz, feldspar, and biotite segregate into layers during regional tectonic events.
Uses & applications
Used in construction as crushed stone for road bases, as a building stone for facades (facing stone), and sometimes in decorative landscaping or as dimension stone for countertops.
Geological facts
Gneiss is one of the oldest rock types found on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The name comes from the German word 'gneist', meaning to spark or glisten.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers. It is ubiquitous in continental shields and mountain ranges. Collectors look for pieces with sharp mineral segregation or containing accessory minerals like garnet.
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