metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs); Color: Alternating dark and light bands (grey, black, white, tan); Luster: Non-metallic, dull to slightly sparkling due to mica; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: None, but splits roughly along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating dark and light bands (grey, black, white, tan)
- Luster
- Non-metallic, dull to slightly sparkling due to mica
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating dark and light bands (grey, black, white, tan); Luster: Non-metallic, dull to slightly sparkling due to mica; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: None, but splits roughly along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. Most gneiss is Precambrian in age, often billions of years old, forming in the deep roots of mountain ranges.
Uses & applications
Used primarily as crushed stone in construction, road base, and landscaping. High-quality slabs are used for countertops, flooring, and decorative building facades.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The term 'gneiss' comes from a German word meaning 'sparkling'.
Field identification & locations
Identify by looking for 'segregated' bands where minerals like quartz/feldspar (light) and biotite/hornblende (dark) form distinct layers. Commonly found in planetary shields and eroded mountain belts. Collectors look for unique contorted folding in the bands.
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