Metamorphic
Granite Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Salt and pepper (gray, white, black, and pink); Luster: Dull to vitreous; Crystal Structure: Coarse-grained phaneritic with distinct foliation (banding); Cleavage: Distinct alignment of minerals but lacks rock cleavage (fractures irregularly); Specific gravity: 2.6 - 2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt and pepper (gray, white, black, and pink)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Salt and pepper (gray, white, black, and pink); Luster: Dull to vitreous; Crystal Structure: Coarse-grained phaneritic with distinct foliation (banding); Cleavage: Distinct alignment of minerals but lacks rock cleavage (fractures irregularly); Specific gravity: 2.6 - 2.9.
Formation & geological history
Formed from the high-grade regional metamorphism of pre-existing igneous rocks (typically granite) or sedimentary rocks under extreme pressure and temperature conditions (400-700°C). Often associated with ancient mountain-building events (orogenies) occurring hundreds of millions to billions of years ago.
Uses & applications
Extensively used in construction for dimensional stone, flooring, countertops, and grave markers; also used as crushed stone for road bases and concrete aggregate. Highly valued by collectors for specimens showing dramatic 'ptygmatic' folding or distinct color banding.
Geological facts
Gneiss forms some of the oldest crustal rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German word meaning 'sparkling' or 'to glow.'
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light-colored felsic minerals (quartz/feldspar) and dark-colored mafic minerals (biotite/amphibole). It is found globally in the stable interiors of continents known as shields. Collectors should look for transitions between granitic and banded textures.
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