metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, tan, or white with distinct dark bands (foliation); Luster: Sub-vitreous to dull; Crystal structure: Coarse-grained, crystalline; Cleavage: Generally none, breaks irregularly.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, or white with distinct dark bands (foliation)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, or white with distinct dark bands (foliation); Luster: Sub-vitreous to dull; Crystal structure: Coarse-grained, crystalline; Cleavage: Generally none, breaks irregularly.
Formation & geological history
Formed by high-pressure and high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). These rocks often date back hundreds of millions to billions of years.
Uses & applications
Used extensively in the construction industry as crushed stone for road bases and as dimension stone for building facades, flooring, and landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is among the oldest known crustal rocks at over 4 billion years old. The term comes from the German word 'Gneis' meaning 'sparked'.
Field identification & locations
Identify by looking for distinct banding (gneissic foliation) where mineral grains are segregated into layers. It is ubiquitous in continental shields and mountain cores. For collectors, look for specimens with large garnets or distinct 'augen' (eye-shaped) structures.
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