metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, gray, and black; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture (gneissic banding); Cleavage: Poor, typically breaks along grain boundaries.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, white, gray, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, gray, and black; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture (gneissic banding); Cleavage: Poor, typically breaks along grain boundaries.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Subjected to intense heat and pressure, causing minerals to segregate into distinct layers. Found in the deeply eroded cores of mountain ranges and ancient shield areas.
Uses & applications
Used as a crushed stone for road construction, landscaping, and concrete aggregate. High-quality slabs are used for countertops, flooring, and decorative building stone.
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 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified by its characteristic 'gneissic banding' where dark and light minerals form alternating layers. Common in continental shields like the Canadian Shield. Look for alternating bands of quartz/feldspar and biotite/amphibole.
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