metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands, often black/grey and white/pink; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation known as 'gneissic banding'; Cleavage: None (unlike schist, it does not split easily along planes).
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands, often black/grey and white/pink; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation known as 'gneissic banding'; Cleavage: None (unlike schist, it does not split easily along planes).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone) subjected to intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Commonly used as a dimension stone for construction, flooring, gravestones, and kitchen countertops. In industry, it is crushed for use as aggregate in road building and railroad ballast.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss in Canada dating back over 4 billion years. The word 'gneiss' comes from an Old German verb meaning 'to spark' because the rock glitters.
Field identification & locations
Identify in the field by looking for 'banding'—distinct layers of different colored minerals that are not perfectly flat but may be folded or contorted. It is commonly found in the cores of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachian Mountains.
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