metamorphic
Gneiss with Orthoclase Feldspar
Gneissic Metamorphic Rock with K-Feldspar inclusions
Hardness: 6-7 (Mohs scale); Color: Black, gray, pink/orange, and white; Luster: Pearly to Vitreous; Crystal structure: Foliated, high-grade crystalline; Cleavage: Good in two directions for the feldspar component.
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, gray, pink/orange, and white
- Luster
- Pearly to Vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, gray, pink/orange, and white; Luster: Pearly to Vitreous; Crystal structure: Foliated, high-grade crystalline; Cleavage: Good in two directions for the feldspar component.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. The pink parts are likely billion-year-old potassium feldspar crystallizing within the band.
Uses & applications
Commonly used as crushed stone for road construction, landscaping, building veneers, and as polished decorative river stones or pocket stones.
Geological facts
The banding seen in gneiss relates to 'gneissic banding' where minerals segregate into layers during intense pressure. Some of the oldest rocks on Earth are types of gneiss like the Acasta Gneiss.
Field identification & locations
Identify in the field by looking for 'salt and pepper' banding and distinct layers of different minerals. Frequently found in glacially deposited riverbeds and mountain ranges.
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