metamorphic
Gneiss
High-grade Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and brown; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded gray, white, and brown
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and brown; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. This creates the characteristic 'gneissic banding' or foliation visible in the specimen.
Uses & applications
Used primarily as a dimension stone for flooring, facing, and architectural ornaments. Also used as crushed stone in construction and road building.
Geological facts
Gneiss is some of the oldest rock on Earth, with some specimens from the Acasta Gneiss in Canada dating back over 4 billion years. The banding is caused by the separation of minerals into layers.
Field identification & locations
Identify in the field by looking for distinct alternating layers (banding) of different colored minerals. It is commonly found in the roots of mountain ranges and ancient continental shields.
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