metamorphic
Banded Gneiss with Quartz Vein
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Black, grey, and white banding; Luster: Vitreous to dull; Crystal structure: Foliated, coarse-grained; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, grey, and white banding
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, grey, and white banding; Luster: Vitreous to dull; Crystal structure: Foliated, coarse-grained; Specific gravity: 2.6-2.9.
Formation & geological history
Formed at extremely high temperatures and pressures deep within the Earth's crust, causing the minerals to segregate into distinct bands (foliation). The white center is a quartz vein formed by hydrothermal fluids filling a fracture later in its history.
Uses & applications
Used primarily as crushed stone for road construction, landscaping, and architectural facing. High-quality specimens are used for countertops and floor tiles.
Geological facts
Gneiss can be some of the oldest rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an old German word meaning 'sparkling'.
Field identification & locations
Identify by looking for distinct 'gneissic banding' where light and dark minerals form parallel layers. Often found in mountainous regions or glacial deposits. Common throughout the Appalachians, Canadian Shield, and Alps.
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