metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded appearance with white, pink (feldspar/quartz) and dark grey/black (biotite/amphibole); Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture with distinct 'gneissose' banding; Cleavage: Generally poor, though individual mica grains show cleavage.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded appearance with white, pink (feldspar/quartz) and dark grey/black (biotite/amphibole); Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture with distinct 'gneissose' banding; Cleavage: Generally poor, though individual mica grains show cleavage.
Formation & geological history
Formed from the regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under extreme temperatures and pressures deep within the Earth's crust. It is often found in the cores of mountain ranges and stable continental shields (cratons).
Uses & applications
Extensively used as a dimension stone for architectural facings, flooring, and gravestones. It is also crushed for use as road aggregate, railroad ballast, and in general construction. High-quality specimens with unique banding are popular among rock collectors.
Geological facts
Gneiss contains some of the oldest rock sequences on Earth, such as the Acasta Gneiss in Canada, which has been dated to approximately 4 billion years old. The name comes from the Middle High German verb 'gneist' meaning to spark, because the rock glitters.
Field identification & locations
Identify in the field by looking for distinct bands of light and dark minerals (foliation) where the bands are thicker and more continuous than in schist. It will not easily split along its layers like slate or schist. Frequently found in glacial till or mountainous regions.
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