metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Grey, white, pink, or green bands; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, white, pink, or green bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, white, pink, or green bands; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. It forms at high temperatures and pressures deep within the Earth's crust.
Uses & applications
Primarily used as a crushed stone in construction, for road base, and as decorative dimension stone in countertops, flooring, and curbing.
Geological facts
Gneiss often contains the oldest minerals on Earth, such as Zircon. Some of the oldest known rock formations in the world, including the Acasta Gneiss in Canada, are over 4 billion years old.
Field identification & locations
Identify by the presence of 'gneissic banding' where light and dark minerals separate into layers. Commonly found in the cores of mountain ranges and ancient continental shields. Looking for alternating quartz/feldspar and mica bands is key.
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