metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Alternating dark and light bands (foliation); Luster: Dull to vitreous; Structure: Coarse-grained with gneissic banding; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating dark and light bands (foliation)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark and light bands (foliation); Luster: Dull to vitreous; Structure: Coarse-grained with gneissic banding; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) under intense heat and pressure deep within the crust. Many specimens are billions of years old from Precambrian shields.
Uses & applications
Used in construction for crushed stone, road building, and architectural facing. Polished specimens are used for countertops, floor tiles, and as decorative landscaping stones.
Geological facts
Gneiss is one of the oldest known rock types on Earth. The Acasta Gneiss in Canada is approximately 4 billion years old, making it some of the oldest crustal material ever discovered.
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding' where minerals are segregated into dark and light layers. Common in mountainous regions and continental shields. Collectors should look for sharp, high-contrast bands.
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