metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, green, and pink banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated texture with distinct mineral layers; Cleavage: Poor, splits along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, green, and pink banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, green, and pink banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated texture with distinct mineral layers; Cleavage: Poor, splits along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (protoliths) under intense heat and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Used as a decorative stone in architecture, flooring, gravitate, and crushed stone for road construction and landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from an Old German word meaning 'sparkling' or 'to glisten'.
Field identification & locations
Identified by 'gneissic banding'—alternating layers of light (quartz/feldspar) and dark (biotite/amphibole) minerals. Commonly found in the crystalline cores of mountain ranges and stable continental shields.
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