metamorphic
Gneiss (with Quartz-Feldspar vein)
Gneiss (high-grade metamorphic rock) containing Quartz (SiO2) and Feldspar (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8)
Hardness: 6-7 on Mohs scale; Color: Gray, tan, white, and black; Luster: Dull to vitreous; Crystal structure: Phaneritic (coarse-grained) with gneissic banding; Specific gravity: 2.6–2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, white, and black; Luster: Dull to vitreous; Crystal structure: Phaneritic (coarse-grained) with gneissic banding; Specific gravity: 2.6–2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths. The depicted specimen shows a segregated mineral vein or 'layering' likely formed during intense heat and pressure deep within the Earth's crust.
Uses & applications
Used primarily in the construction industry as crushed stone for road base, aggregate, and as 'granite' for countertops, flooring, and dimensional stone.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify by 'gneissic banding' where minerals are separated into distinct light and dark layers. Found globally in the cores of mountain ranges and ancient shielding areas (e.g., Canadian Shield, Appalachians). Collectors look for distinct mineral segregation.
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