metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white, gray, and tan layers; Luster: Dull to vitreous; Crystal structure: Crystalline with coarse mineral grains; Cleavage: Poorly defined but splits along foliation; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded appearance with white, gray, and tan layers
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white, gray, and tan layers; Luster: Dull to vitreous; Crystal structure: Crystalline with coarse mineral grains; Cleavage: Poorly defined but splits along foliation; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale/sandstone). It is often Precambrian in age but can form in any mountain-building event.
Uses & applications
Used as a dimension stone for flooring, countertops, and facing stone; also crushed for use as aggregate in construction and road building.
Geological facts
Gneiss is some of the oldest rock found on Earth, such as the Acasta Gneiss in Canada, dated to approximately 4 billion years old. The banding is caused by the segregation of minerals into dark-colored (mafic) and light-colored (felsic) layers.
Field identification & locations
Identify in the field by the distinct 'gneissose' banding—alternating light and dark layers of minerals. Commonly found in the cores of mountain ranges or deeply eroded shield areas. It is an extremely common but beautiful specimen for amateur collectors.
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