metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded gray, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded gray, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded gray, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous (granite) or sedimentary (shale/sandstone) rocks under extreme heat and pressure. Most specimens range from hundreds of millions to billions of years old.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and as dimension stone for building facades, floor tiles, and gravestones. Valuable for its durability and aesthetic banding.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. It is distinguished from schist by its lack of distinct foliation planes and thicker banding.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers. Commonly found in the crystalline cores of mountain ranges and eroded continental shields. Collectors look for unique swirling patterns or eye-shaped mineral structures called 'augen'.
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