metamorphic
Granitic Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Pink, grey, and black banding; Luster: Dull to vitreous; Crystal Structure: Coarse-grained phaneritic; Cleavage: Poor, but exhibits foliation-based fracturing; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, grey, and black banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, grey, and black banding; Luster: Dull to vitreous; Crystal Structure: Coarse-grained phaneritic; Cleavage: Poor, but exhibits foliation-based fracturing; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under extreme heat and pressure. This specific sample likely dates back to the Proterozoic or Archean eras in many shield regions.
Uses & applications
Used primarily in heavy construction as crushed stone or road base, and as dimension stone for building facades, countertops, and landscaping.
Geological facts
Gneiss is among the oldest known rocks on Earth, including the Acasta Gneiss in Canada, which is roughly 4 billion years old. The term comes from a Middle High German word meaning 'sparkle'.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—alternating layers of light (felsic) and dark (mafic) minerals. Commonly found in the roots of mountain ranges and ancient continental shields like the Canadian Shield.
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