metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded appearance; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, foliated (gneissose banding); Cleavage: Poor, but splits along bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded appearance
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded appearance; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, foliated (gneissose banding); Cleavage: Poor, but splits along bands; 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) under extreme heat and pressure, often during mountain-building events. Geological age varies from Archean to Cenozoic.
Uses & applications
Used as a dimension stone for flooring, facing, and gravestones; also crushed for use in road construction and as concrete aggregate. Occasionally used as decorative landscaping stone.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The term comes from the German word 'Gneis', meaning 'spark'.
Field identification & locations
Identify by its characteristic 'salty and pepper' appearance with distinct light and dark mineral bands (foliation). Found globally in the roots of eroded mountain ranges and continental shields. Look for alternating bands of quartz/feldspar (light) and biotite/hornblende (dark).
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