metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs); Color: Gray, green, and white with bands; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs)
- Color
- Gray, green, and white with bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Gray, green, and white with bands; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
Formation & geological history
Formed by the high-pressure, high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). The distinct banding, known as gneissic foliation, occurs when minerals recrystallize and migrate into separate layers.
Uses & applications
Commonly used in construction as crushed stone for road bases and concrete, or as dimension stone for counter tops, flooring, and gravestones. Used as an ornamental stone in landscaping.
Geological facts
Gneiss can be among the oldest rocks on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The banding often represents the extreme folding and deformation the rock underwent deep within the crust.
Field identification & locations
Look for alternating light and dark bands (gneissic banding) and a coarse, granular texture. Commonly found in the roots of mountain ranges (crystalline massifs) across North America and Europe. Collectors look for interesting folding patterns or garnet inclusions.
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