metamorphic
Granitic Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: light tan, grey, and pinkish bands; Luster: dull to vitreous; Structure: foliated with visible crystalline layers (gneissic banding); Cleavage: none.
- Hardness
- 6-7 (Mohs scale)
- Color
- light tan, grey, and pinkish bands
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: light tan, grey, and pinkish bands; Luster: dull to vitreous; Structure: foliated with visible crystalline layers (gneissic banding); Cleavage: none.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of igneous granite or sedimentary protoliths. This specimen shows a weathered, rounded exterior typical of fluvial or alluvial transport.
Uses & applications
Used primarily in construction as crushed stone or 'rip rap,' architecture as dimension stone for flooring or countertops, and as decorative landscaping stones.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens (Acasta Gneiss) dating back over 4 billion years. The banding occurs because minerals segregate during crystallization under intense pressure.
Field identification & locations
Identify in the field by looking for distinct parallel 'stripes' or banding of different minerals (usually quartz, feldspar, and mica). Found globally in continental shields and mountain ranges.
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