metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, tan, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, tan, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, tan, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9.
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. It is often associated with mountain-building events (orogenies).
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality specimens are used as dimension stone for flooring, countertops, and grave markers.
Geological facts
Gneiss contains some of the oldest rocks found on Earth, such as the Acasta Gneiss in Canada, which is roughly 4 billion years old. The term comes from a Middle High German verb meaning 'to spark'.
Field identification & locations
Identify it by looking for its characteristic 'gneissic banding', where minerals are segregated into alternating light and dark layers. It is ubiquitous in the roots of mountain ranges and shield areas like the Canadian Shield or the Appalachian Mountains.
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