metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray with white and black bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray with white and black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray with white and black bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to intense heat and pressure, often during mountain-building events.
Uses & applications
Used as a crushed stone for road construction, landscaping, and as a decorative building stone (granite-like countertops or floor tiles).
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The banding occurs because intense pressure causes different minerals to migrate into layers.
Field identification & locations
Identify in the field by looking for 'salty and pepper' coloring arranged in distinct alternating light and dark bands. Commonly found in the crystalline cores of mountain ranges and ancient continental shields.
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