metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: alternating light (quartz/feldspar) and dark bands; Luster: Vitreous to dull; Crystal Structure: Coarse-grained foliated; Cleavage: Poor, though it can split along foliation layers.
- Hardness
- 6-7 (Mohs scale)
- Color
- alternating light (quartz/feldspar) and dark bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: alternating light (quartz/feldspar) and dark bands; Luster: Vitreous to dull; Crystal Structure: Coarse-grained foliated; Cleavage: Poor, though it can split along foliation layers.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. This specimen shows 'gneissic banding' and appears water-worn (rounded) from river or glacial activity.
Uses & applications
Commonly used as crushed stone for road construction, as architectural veneer, flooring, and decorative landscaping stones.
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding occurs due to the segregation of minerals during recrystallization.
Field identification & locations
Identify in the field by looking for 'schistosity' or clear compositional banding (stripes) and a coarse texture. It will not fizz with acid (unlike marble) and is very hard.
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