metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white alternating bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white alternating bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white alternating bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. This process often takes millions of years and occurs deep within the Earth's crust.
Uses & applications
Commonly used in construction as crushed stone for roads, as dimension stone for building facades, countertops, and occasionally as decorative landscaping stones.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The banding is caused by the segregation of light and dark minerals during intense tectonic activity.
Field identification & locations
Identify by looking for 'gneissic banding'—alternating layers of light (quartz/feldspar) and dark (biotite/hornblende) minerals. Found globally in mountainous regions or ancient continental shields. For collectors, water-tumbled specimens like this one are common on riverbanks.
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