metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs); Color: Alternating light (white/tan/pink) and dark (black) bands; Luster: Vitreous to dull; Crystal structure: Crystalline, showing coarse foliation (gneissic banding); Cleavage: None (fractures irregularly); Specific gravity: 2.6 - 2.9.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating light (white/tan/pink) and dark (black) bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating light (white/tan/pink) and dark (black) bands; Luster: Vitreous to dull; Crystal structure: Crystalline, showing coarse foliation (gneissic banding); Cleavage: None (fractures irregularly); Specific gravity: 2.6 - 2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. This process typically occurs deep within tectonic mountain-building zones (orogenic belts) over millions of years.
Uses & applications
Used primarily in the construction industry as crushed stone for road aggregate and railroad ballast. High-quality decorative slabs are polished for use as countertops, flooring, and ornamental building stone.
Geological facts
Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. Its unique banding is called 'gneissic banding,' which results from the segregation of mineral grains during metamorphism.
Field identification & locations
Identify in the field by looking for 'salt and pepper' appearances and distinct layering or banding. Common in shields of continents (like the Canadian Shield). Collectors look for specimens with sharp color contrasts or folding (pTYgmatic folds).
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