metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pinkish-tan with dark grey/black bands; Luster: Dull to vitreous; Crystal Structure: Foliated with distinct banding (gneissic foliation); Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-tan with dark grey/black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-tan with dark grey/black bands; Luster: Dull to vitreous; Crystal Structure: Foliated with distinct banding (gneissic foliation); Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under high temperature and pressure. Often Precambrian in age.
Uses & applications
Used as a crushed stone for road construction, as dimension stone for architectural facings and flooring, and as landscaping material.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' meaning 'to spark'.
Field identification & locations
Identified by alternating bands of light and dark minerals (foliation). Look for coarse grains and visible layering without easy split-ability like schist. Found globally in the cores of mountain ranges and ancient continental shields.
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