metamorphic
Gneiss
Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture with distinct mineral banding; Cleavage: Generally poor, though mica layers may split.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture with distinct mineral banding; Cleavage: Generally poor, though mica layers may split.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). This process typically occurs at convergent plate boundaries under extreme pressure and temperature conditions (typically deep in the earth's crust).
Uses & applications
Used in construction for road aggregates, building stones, and decorative facing. Higher-quality specimens with attractive banding are used for countertops, curbing, and ornamental landscaping stones.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified in the field by its 'gneissic banding'—alternating layers of light-colored (felsic) minerals like quartz or feldspar and dark-colored (mafic) minerals like biotite or hornblende. It is common in the cores of mountain ranges and stable continental cratons globally.
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