metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray with distinct dark and light colored banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray with distinct dark and light colored banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray with distinct dark and light colored banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. This specimen displays characteristic 'gneissic banding' caused by mineral segregation during deep crustal deformation.
Uses & applications
Commonly used as a decorative stone in architecture, for flooring, gravestones, and as crushed stone for road construction and landscaping due to its durability.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The rhythmic light and dark bands are a classic example of metamorphic layering.
Field identification & locations
Identify in the field by looking for 'gneissic banding,' which consists of alternating layers of light-colored minerals (like quartz or feldspar) and dark-colored minerals (like biotite or hornblende). It is commonly found in mountain ranges and glacial erratic fields.
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