metamorphic
Banded Gneiss
Gneiss (High-grade metamorphic rock)
Distinct alternating layers of light and dark minerals (gneissic banding). Hardness: 6-7 on Mohs scale. Colors: Gray, white, pink, and black. Luster: Dull to vitreous. Structure: Foliated with medium to coarse grains. Cleavage: Poor, but breaks along bands.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Distinct alternating layers of light and dark minerals (gneissic banding). Hardness: 6-7 on Mohs scale. Colors: Gray, white, pink, and black. Luster: Dull to vitreous. Structure: Foliated with medium to coarse grains. Cleavage: Poor, but breaks along bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. This causes minerals to recrystallize and segregate into bands. Often dating back to the Precambrian era.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and as dimension stone for architectural facades, countertops, and flooring. Occasionally used in landscaping.
Geological facts
Gneiss is one of the oldest known rocks on Earth; the Acasta Gneiss in Canada is over 4 billion years old. The banding is often contorted or folded, showcasing the extreme tectonic forces the rock endured.
Field identification & locations
Identify by looking for 'gneissose' banding where minerals are segregated into dark (biotite/amphibole) and light (quartz/feldspar) stripes. Commonly found in the cores of mountain ranges and ancient continental shields. Collectors look for beautiful folding patterns.
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