metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, white, pink, or multicolored with distinct alternating light and dark bands (foliation/gneissic banding); Luster: Dull to sub-vitreous; Crystal Structure: Crystalline, granular; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, pink, or multicolored with distinct alternating light and dark bands (foliation/gneissic banding); Luster: Dull to sub-vitreous; Crystal Structure: Crystalline, granular; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism, typically from preexisting igneous rocks (like granite or diorite) or sedimentary rocks (like shale or sandstone) subjected to intense heat and directed pressure deep within the Earth's crust.
Uses & applications
Used primarily as crushed stone in construction, building facades, paving, curbing, and occasionally as decorative dimension stone.
Geological facts
Gneiss includes some of the oldest rocks found on Earth, with certain formations dating back over 4 billion years, such as the Acasta Gneiss in Canada.
Field identification & locations
Easily identified in the field by its distinct alternating light and dark compositional bands (gneissic banding). Commonly found in ancient shields and core areas of mountain ranges. Location data (East Africa) is entirely consistent with ancient metamorphic basement rocks.
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