metamorphic
Gneiss
Gneissic Metamorphic Rock (Mineral composition: Quartz, Feldspar, Amphibole/Biotite)
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, tan, or tan-orange with dark bands; Luster: dull to vitreous; Structure: foliated with distinct banding (gneissic banding); Cleavage: none (fractures across bands).
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, tan, or tan-orange with dark bands; Luster: dull to vitreous; Structure: foliated with distinct banding (gneissic banding); Cleavage: none (fractures across bands).
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths. It forms at extreme temperatures and pressures where minerals recrystallize and segregate into alternating light and dark layers.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete, or as dimension stone for architectural facades, countertops, and flooring. Highly durable for outdoor landscaping.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is roughly 4.03 billion years old. The term comes from a Middle High German verb 'gneist', meaning to spark, because the rock glitters.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating bands of different colors (mineral segregation) without the fine-grained breakage of schist. Commonly found in the crystalline cores of mountain ranges or in glacial tills. Collectors look for pieces with contorted folds or garnet inclusions.
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