metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black; Luster: Dull to sub-vitreous; Crystal structure: Crystalline with visible mineral grains; Cleavage: Poor, but exhibits systematic foliation; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, and black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black; Luster: Dull to sub-vitreous; Crystal structure: Crystalline with visible mineral grains; Cleavage: Poor, but exhibits systematic foliation; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). Subjected to high temperature and pressure, causing minerals to segregate into bands. Often Precambrian in age.
Uses & applications
Used as crushed stone in construction, road base, and occasionally as decorative stone or countertops when the banding is particularly aesthetic.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old. The banding is specifically known as 'gneissic banding.'
Field identification & locations
Identify in the field by looking for distinct 'foliation' or alternating bands of light (quartz/feldspar) and dark (biotite/hornblende) minerals. Found globally in mountainous regions and continental shields. Does not split easily along planes like slate.
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