metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Distinct alternating dark (biotite, hornblende) and light (quartz, feldspar) bands; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissic banding); Cleavage: Generally poor, though individual mica flakes within stripes have perfect cleavage; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Distinct alternating dark (biotite, hornblende) and light (quartz, feldspar) bands; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissic banding); Cleavage: Generally poor, though individual mica flakes within stripes have perfect cleavage; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous (orthogneiss) or sedimentary (paragneiss) rocks. This occurs under intense heat and pressure deep within the Earth's crust, often during mountain-building (orogenic) events, dating back millions to billions of years.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality slabs are used for countertops, flooring, and decorative building facades. It is also common in landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The name comes from a German word meaning 'sparkling' or 'to glow' due to the presence of mica.
Field identification & locations
Identify in the field by its characteristic 'gneissic banding'—alternating layers of different mineral compositions. It is widely found in the roots of eroded mountain chains and ancient continental shields (like the Canadian Shield or Scandinavia). Collectors look for specimens with Sharp, contrasting colors or garnet inclusions.
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