metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black bands; Luster: Variable, usually dull to vitreous; Crystal structure: Coarse-grained with distinct foliation; Cleavage: Poorly defined; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black bands
- Luster
- Variable, usually dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black bands; Luster: Variable, usually dull to vitreous; Crystal structure: Coarse-grained with distinct foliation; Cleavage: Poorly defined; 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. This occurs at high temperatures and pressures deep within the Earth's crust, often during mountain-building events.
Uses & applications
Commonly used as crushed stone in road construction and landscaping. High-quality slabs are used for building facades, countertops, and decorative floor tiles due to its durability and attractive banding.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is over 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified by 'gneissose banding', which is the separation of light (quartz/feldspar) and dark (biotite/hornblende) minerals into distinct layers. Found in the cores of many mountain ranges and stable continental cratons.
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