metamorphic
Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, pink, and black; Luster: Pearly to dull; Texture: Coarse-grained with distinct foliation (layering); Composition: Quartz, feldspar, and dark minerals like biotite or hornblende.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, pink, and black
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, pink, and black; Luster: Pearly to dull; Texture: Coarse-grained with distinct foliation (layering); Composition: Quartz, feldspar, and dark minerals like biotite or hornblende.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, often found in the cores of mountain ranges.
Uses & applications
Commonly used as a decorative building stone, for flooring, gravestones, and as crushed stone in road construction due to its durability.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is dated at approximately 4.03 billion years old. The banding is caused by the segregation of minerals during metamorphism.
Field identification & locations
Identify by looking for distinct 'gneissic banding', where light and dark minerals form parallel layers. Often found in glaciated regions or areas of deep crustal exposure. Found worldwide in continental crust.
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