metamorphic
Gneiss
Metamorphic Rock (Gneissic texture)
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating dark and light layers (gray, pink, white, black); Luster: dull to vitreous; Crystal structure: crystalline but with a foliated, banded texture; Cleavage: poor; Specific gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating dark and light layers (gray, pink, white, black); Luster: dull to vitreous; Crystal structure: crystalline but with a foliated, banded texture; Cleavage: poor; Specific gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone) under intense heat and pressure deep in the Earth's crust. Most exposures are from the Precambrian or Paleozoic eras.
Uses & applications
Used as a dimension stone for architectural facades, flooring, and curbing. Also crushed for use in road construction and as concrete aggregate.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, dated to about 4 billion years. It is defined by its 'gneissic banding' where minerals segregate into distinct layers.
Field identification & locations
Identify in the field by looking for distinct, thick bands of minerals (foliation) that are usually coarser than schist. It is very common in mountain ranges like the Rockies or Alps where deep-seated crustal material has been uplifted and eroded.
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