metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Characteristic alternating bands of pink (feldspar), white (quartz), and grey/black (biotite/amphibole); Luster: Dull to vitreous; Structure: Foliated with 'gneissose' banding; Cleavage: Poor, but breaks along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Characteristic alternating bands of pink (feldspar), white (quartz), and grey/black (biotite/amphibole); Luster: Dull to vitreous; Structure: Foliated with 'gneissose' banding; Cleavage: Poor, but breaks along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure. This specimen likely dates back to the Precambrian or Paleozoic eras depending on the relative local geology.
Uses & applications
Used in construction as crushed stone for road bases and concrete, or as architectural 'dimension stone' for countertops, flooring, and gravestones. It is often sold as 'granite' in the commercial countertop industry.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. It displays a distinctive texture where minerals separate into dark and light layers based on density and chemical composition.
Field identification & locations
Identify in the field by looking for distinct mineral banding (foliation) where the minerals are coarse enough to be seen with the naked eye, unlike schist. It is commonly found in the 'crystalline basements' of mountain ranges like the Appalachians or Alps.
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