metamorphic
Gneiss
Gneiss (Metamorphic high-grade)
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary precursor rocks (protoliths) under intense heat and pressure deep in the Earth's crust.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for building facades, countertops, and decorative landscaping.
Geological facts
The Gneissic banding is caused by the segregation of different mineral types into light (quartz/feldspar) and dark (biotite/hornblende) layers during metamorphism.
Field identification & locations
Identify by looking for 'gneissic banding' or distinct alternating color stripes. Commonly found in the cores of mountain ranges and ancient crystalline shields like the Canadian Shield.
