metamorphic
Granitic Gneiss
Gneiss (Polymetamorphic Granitic Gneiss)
Hardness: 6-7 on Mohs scale. Color: Pink (Orthoclase Feldspar), White/Grey (Quartz), and Black (Biotite/Hornblende). Luster: Dull to vitreous. Structure: Gneissic banding (foliation) where light and dark minerals are separated into distinct layers. Texture: Medium to coarse-grained.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Pink (Orthoclase Feldspar), White/Grey (Quartz), and Black (Biotite/Hornblende). Luster: Dull to vitreous. Structure: Gneissic banding (foliation) where light and dark minerals are separated into distinct layers. Texture: Medium to coarse-grained.
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite. Subjected to intense heat and pressure deep within the Earth's crust, causing minerals to recrystallize and align into bands. Many specimens are billions of years old, often associated with Precambrian shield regions.
Uses & applications
Used as crushed stone in construction, road base, and concrete aggregate. High-quality slabs are used for countertops, flooring, and gravestones. It is also collected by geology students and hobbyists as an example of foliation.
Geological facts
Gneiss is some of the oldest rock found on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. Because of its intense formation process, it rarely contains fossils.
Field identification & locations
Identify in the field by looking for 'zebra stripes' or distinct alternating bands of light and dark minerals. Common in glacial till across the Midwest USA, Canada, and Scandinavia. Unlike schist, it does not split easily along its planes.
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