metamorphic
Granitic Gneiss
Gneiss (Metamorphic rock of granitic composition)
Hardness: 6-7 (Mohs scale); Color: Pink to reddish with dark grey or black bands; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Composition: Quartz, Potassium Feldspar, and Biotite/Amphibole.
- Hardness
- 6-7 (Mohs scale)
- Color
- Pink to reddish with dark grey or black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Pink to reddish with dark grey or black bands; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Composition: Quartz, Potassium Feldspar, and Biotite/Amphibole.
Formation & geological history
Formed through high-grade regional metamorphism of igneous rocks like granite. This specific rounded specimen suggests it was shaped by fluvial (water) or glacial erosion over thousands to millions of years.
Uses & applications
Used primarily in construction as crushed stone, dimension stone for countertops and flooring, and occasionally as decorative landscaping stones or river rocks.
Geological facts
The banding in gneiss, known as gneissic foliation, occurs because minerals segregate into layers under intense heat and pressure. Gneiss often represents some of the oldest rock formations on the Earth's crust.
Field identification & locations
Identify in the field by looking for distinct alternating bands of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/hornblende). It is commonly found in the cores of mountain ranges.
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