metamorphic
Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Alternating dark and light bands (grey, black, pink, white). Luster: Dull to vitreous. Texture: Medium to coarse-grained with characteristic gneissic foliation (banding).
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating dark and light bands (grey, black, pink, white)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Alternating dark and light bands (grey, black, pink, white). Luster: Dull to vitreous. Texture: Medium to coarse-grained with characteristic gneissic foliation (banding).
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like sandstone/shale) under extreme temperatures and pressures, causing distinct mineral segregation into light and dark bands.
Uses & applications
Widely used as crushed stone in construction, road base, and ballast. Highly durable specimens are used for building facade stones, curbstones, paving blocks, and landscaping.
Geological facts
Gneiss contains some of the oldest rocks on Earth, including the Acasta Gneiss in Northwest Territories, Canada, which is approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for distinct, non-continuous alternating mineral bands (gneissic banding) of quartz/feldspar and dark ferromagnesian minerals like biotite or hornblende. Common in ancient crystalline terranes worldwide.
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