metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with distinct foliation (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with distinct foliation (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme heat and pressure. Commonly found in the deep crustal roots of mountain ranges (orogenic belts) and ancient continental shields.
Uses & applications
Used as a crushed stone in construction for road foundations, concrete aggregate, and railroad ballast. High-quality slabs are used for countertops, floor tiles, and as ornamental building stone.
Geological facts
Gneiss is one of the oldest rock types on Earth, with the Acasta Gneiss in Canada dating back approximately 4 billion years. The distinct light and dark bands are a result of 'metamorphic differentiation' where minerals separate at high temperatures.
Field identification & locations
Identify by the characteristic 'gneissose banding' of alternating light (quartz/feldspar) and dark (biotite/amphibole) minerals. Often found in glaciated regions as erratic boulders or in exposed mountain outcrops. Look for coarse grains and a lack of distinct cleavage planes common in schist.
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