metamorphic
Gneiss (Granitic Gneiss)
Gneiss (composed primarily of Quartz, Feldspar, and Mica)
Hardness: 6-7 (Mohs scale); Color: Gray, pink, white, and tan bands; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor, but breaks along foliation planes; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, pink, white, and tan bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, pink, white, and tan bands; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor, but breaks along foliation planes; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks under extreme pressure and temperature. This specimen's rounded shape indicates recent erosion in a river or glacial environment.
Uses & applications
Used as a decorative crushed stone, dimension stone for construction, paving blocks, and occasionally for landscaping or as a collector's specimen of metamorphic processes.
Geological facts
The banding in gneiss is called 'gneissic foliation' and results from the segregation of light (felsic) and dark (mafic) minerals at high temperatures. Some of the oldest known rocks on Earth are the Acasta Gneiss in Canada, dated to 4 billion years ago.
Field identification & locations
Identify in the field by looking for distinct, thick bands of alternating colors and a crystalline texture. It is commonly found in the roots of mountain ranges (cratons) and as glacial till or river stones in previously glaciated regions.
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