metamorphic
Banded Gneiss
Gneiss (Metamorphic silicate rock)
Hardness: 6-7 (Mohs scale); Color: Grayish-white with dark black/blue bands; Luster: Pearly to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Grayish-white with dark black/blue bands
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Grayish-white with dark black/blue bands; Luster: Pearly to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths. Under high temperature and pressure, minerals segregate into bands. Specific age varies by region, often Proterozoic or Archean.
Uses & applications
Used as crushed stone for road construction, building stone for countertops and flooring, and occasionally as decorative landscaping stone.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being approximately 4 billion years old.
Field identification & locations
Identify by distinctive 'gneissose banding' (alternating light and dark layers). Commonly found in the crystalline cores of mountain ranges and glacial till. Look for interlocking crystals and parallel alignment.
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