metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Variable (dull to vitreous); Crystal Structure: Coarse-grained foliated; Cleavage: Poorly developed, tends to break along foliations; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, and black
- Luster
- Variable (dull to vitreous)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Variable (dull to vitreous); Crystal Structure: Coarse-grained foliated; Cleavage: Poorly developed, tends to break along foliations; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. It often dates back to the Precambrian era.
Uses & applications
Used as a dimension stone for flooring, cemetery markers, facades, and crushed stone for road construction and landscaping.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The distinct banding is called 'gneissose banding'.
Field identification & locations
Identify in the field by looking for 'salt and pepper' appearance with distinct mineral layering (foliation). Found in mountainous regions and continental shields globally. Highly resistant to erosion.
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