metamorphic
Gneiss
High-grade Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, and white banded appearance; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with distinct mineral banding; Cleavage: Poor, typically breaks along foliation planes; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey, black, and white banded appearance
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, and white banded appearance; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with distinct mineral banding; Cleavage: Poor, typically breaks along foliation planes; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense pressure and high temperatures (above 600°C). This process causes minerals to recrystallize and segregate into distinct layers or bands known as gneissic foliation.
Uses & applications
Commonly used in construction as crushed stone for road bases and railway ballast. Polished high-quality specimens are used for countertops, floor tiles, and as facing stone on buildings. It also has value as a decorative garden stone.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada which is approximately 4.03 billion years old. The term comes from the Middle High German verb 'gneist' meaning to spark, because the rock glitters.
Field identification & locations
Identified in the field by its characteristic alternating bands of light and dark minerals (foliation) and large mineral grains. It is extremely common in the deep crust of continental shields. Collectors look for pieces with high contrast between bands or garnet inclusions.
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