metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Grey, white, pink, and black bands; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, white, pink, and black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, white, pink, and black bands; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor; 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) under extreme heat and pressure. It is often found in the cores of mountain ranges and ancient continental shields.
Uses & applications
Used as a dimension stone for flooring, facing on buildings, and gravestones. It is also crushed for use as aggregate in road construction and landscaping.
Geological facts
Gneiss represents one of the highest degrees of metamorphism before a rock begins to melt into magma. Some of the oldest known rocks on Earth are the Acasta Gneiss in Canada, dated to approximately 4 billion years old.
Field identification & locations
Identify by looking for alternating light-colored (feldspar/quartz) and dark-colored (biotite/amphibole) bands. It does not split as easily along planes as schist or slate. It is ubiquitous in glaciated regions as river cobbles or glacial erratics.
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