metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pink, gray, and black banding; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Texture: Medium to coarse-grained.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, and black banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, and black banding; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Texture: Medium to coarse-grained.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure. These specimens often date back to the Precambrian era.
Uses & applications
Used as a dimension stone for flooring, wall facing, and gravestones; also used as crushed stone for road construction and aggregate. Tumbled pieces are popular for educational kits.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens including the Acasta Gneiss dating back over 4 billion years. The name comes from a German word meaning 'sparkling' or 'bright'.
Field identification & locations
Identified by its alternating dark and light colored bands. It can be found globally in the cores of mountain ranges and eroded crystalline shields like the Canadian Shield. For collectors, look for the 'salt and pepper' appearance with layered stripes.
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