metamorphic
Feldspathic Gneiss
High-grade Metamorphic Rock (Gneiss)
Hardness: 6-7 (Mohs scale); Color: Gray, light pink, and white with dark banding; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Composition: Primarily quartz, feldspar, and darker minerals like mica or hornblende.
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, light pink, and white with dark banding
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, light pink, and white with dark banding; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Composition: Primarily quartz, feldspar, and darker minerals like mica or hornblende.
Formation & geological history
Formed under high temperature and high pressure during regional metamorphism of igneous or sedimentary parent rocks. This specimen shows a granitic protolith and likely dates back hundreds of millions to billions of years.
Uses & applications
Commonly used as crushed stone in construction, road base, and occasionally as structural stone or landscaping material. High-quality slabs are used for countertops.
Geological facts
Gneiss is one of the oldest rocks on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The banding, known as gneissic banding, is caused by the segregation of light and dark minerals.
Field identification & locations
Identify in the field by looking for distinct layers or 'stripes' of minerals. It is commonly found in the roots of mountain ranges and precambrian shields worldwide. This piece appears to be a stream-tumbled cobble.
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