metamorphic
Banded Gneiss
Banded Gneiss (Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Black, gray, and white stripes; Luster: Dull to sub-vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor.
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, gray, and white stripes
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, gray, and white stripes; Luster: Dull to sub-vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature, causing minerals to segregate into bands. Often Precambrian in age.
Uses & applications
Used as crushed stone for road construction, as dimension stone for counter tops and flooring, and as decorative landscaping pebbles.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is caused by the separation of light minerals like quartz and feldspar from dark minerals like biotite.
Field identification & locations
Identified by its characteristic light and dark alternating bands. Commonly found in the roots of mountain ranges or in glacial tills. Collectors look for high contrast and sharp line definitions in pebbles.
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