metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Variable with distinct banding (foliation); Luster: Vitreous to dull; Crystal structure: Coarse-grained granular with alternating light and dark layers of minerals like quartz, feldspar, and mica.
- Hardness
- 6-7 on Mohs scale
- Color
- Variable with distinct banding (foliation)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Variable with distinct banding (foliation); Luster: Vitreous to dull; Crystal structure: Coarse-grained granular with alternating light and dark layers of minerals like quartz, feldspar, and mica.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. Often associated with mountain-building events (orogenies) dating from the Archean Eon to the Mesozoic.
Uses & applications
Used primarily in construction for crushed stone, road base, and landscape stone. High-quality specimens with attractive banding are used for countertops, veneers, and gravestones.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. It is distinguished from schist by its lack of a well-defined cleavage that splits the rock evenly.
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding'—alternating bands of different colors (compositional layering). Found globally in continental shields and the roots of eroded mountain ranges.
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