metamorphic
Banded Gneiss
Gneiss (Metamorphic high-grade rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture (gneissic banding); Cleavage: Poorly defined due to interlocking grains.
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture (gneissic banding); Cleavage: Poorly defined due to interlocking grains.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme pressure and temperature. This process leads to recrystallization and the segregation of minerals into bands. Many gneiss formations date from the Precambrian era (over 541 million years ago).
Uses & applications
Predominantly used as crushed stone in road construction and as a decorative building stone. High-quality slabs are polished for use as countertops, flooring tiles, and cemetery monuments.
Geological facts
Gneiss includes some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is roughly 4 billion years old. The term comes from the Middle High German verb 'gneist' meaning 'to spark' because the rock often glitters.
Field identification & locations
Identify in the field by looking for distinct, parallel, rhythmic bands of different minerals that are often folded or contorted. It is commonly found in the cores of mountain ranges and in ancient continental shields (e.g., Canadian Shield, Scandinavian Shield). Collectors look for specimens with dramatic folding.
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