metamorphic
Gneiss
Gneissic Metamorphic Rock
Hardness: 6-7 on Mohs scale; Color: Banded pink, gray, white, and black; Luster: Vitreous to dull; Structure: Gneissic banding (foliation); Cleavage: Generally none, though minerals like mica within it may show cleavage; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, gray, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pink, gray, white, and black; Luster: Vitreous to dull; Structure: Gneissic banding (foliation); Cleavage: Generally none, though minerals like mica within it may show cleavage; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths. It undergoes extreme heat and pressure, often at convergent plate boundaries, dating back to various geological eras including the Precambrian.
Uses & applications
Used primarily as a dimension stone for flooring, gravestones, and facing on buildings. Also used as crushed stone for road construction and landscaping. Occasionally used for artistic carvings or decorative garden rocks.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' (to spark).
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding,' where light and dark minerals are segregated into repetitive layers. It is ubiquitous in the cores of mountain ranges and crystalline shields (like the Canadian Shield). Collectors look for specimens with Sharp color contrasts or unique folding.
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