metamorphic
Gneiss
Gneiss (high-grade metamorphic rock typically composed of quartz, feldspar, and mica)
Hardness: 6-7 (Mohs scale); Color: gray, pink, or white bands alternating with dark biotite or hornblende; Luster: vitreous to dull; Crystal structure: regional metamorphic texture with coarse grains; Cleavage: poor (splits along foliation)
- Hardness
- 6-7 (Mohs scale)
- Luster
- vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: gray, pink, or white bands alternating with dark biotite or hornblende; Luster: vitreous to dull; Crystal structure: regional metamorphic texture with coarse grains; Cleavage: poor (splits along foliation)
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep within the Earth's crust over millions of years.
Uses & applications
Commonly used as crushed stone in construction, road base, and as decorative dimension stone for countertops, flooring, and building facades due to its durability and attractive banding.
Geological facts
Gneiss is one of the oldest rock types found on Earth. The Acasta Gneiss in Canada is approximately 4 billion years old, making it some of the oldest known continental crust.
Field identification & locations
Identify in the field by looking for 'gneissose banding,' which are distinct layers of light and dark minerals. It is commonly found in the cores of mountain ranges and eroded shield areas like the Canadian Shield or the Appalachians.
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