metamorphic
Gneiss (Metamorphosed Granitic Rock)
Gneiss (composed mainly of quartz, feldspar, biotite, and/or hornblende)
Hardness: 6-7 on Mohs scale; Color: Grayish to brownish-tan with subtle dark and light mineral banding; Luster: Dull to vitreous; Texture: Medium to coarse-grained metamorphic texture with weak to moderate foliation.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grayish to brownish-tan with subtle dark and light mineral banding; Luster: Dull to vitreous; Texture: Medium to coarse-grained metamorphic texture with weak to moderate foliation.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (like granite) or sedimentary rocks under high temperature and intense pressure deep within Earth's crust.
Uses & applications
Commonly used as crushed stone for construction aggregate, road base, landscaping rock, and occasionally as dimension stone for building facades or countertops.
Geological facts
Gneiss is one of the oldest rocks found on Earth, with some Canadian shield samples dating back over 4 billion years. Smooth river cobbles of gneiss like this one show natural erosion from water transport.
Field identification & locations
Look for distinct light (feldspar/quartz) and dark (biotite/amphibole) mineral grains arranged in subtle layers or bands. Frequently found along riverbeds, glacial deposits, and mountainous metamorphic terrains.
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