metamorphic
Gneiss
Gneiss (high-grade metamorphic rock)
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct mineral banding (foliation); Cleavage: Resists cleavage, prefers to break unevenly; Specific gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Gray, pink, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct mineral banding (foliation); Cleavage: Resists cleavage, prefers to break unevenly; Specific gravity: 2.6-2.9
Formation & geological history
Formed through the high-temperature and high-pressure metamorphism of granite or sedimentary rocks like shale; typically dates back to the Precambrian or Paleozoic eras within the deep roots of mountain belts.
Uses & applications
Used in construction as crushed stone for road bases, and occasionally as dimension stone for countertops, flooring, and gravestones because of its durability and aesthetic banding.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss dating back nearly 4.03 billion years. The term 'gneiss' comes from an Old German word meaning 'sparked' or 'shining'.
Field identification & locations
Identify in the field by looking for 'gneissose banding' - alternating layers of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). Common in the Appalachian Mountains, Scandinavia, and the Canadian Shield.
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