metamorphic
Gneiss (specifically Granite Gneiss)
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Banded appearance with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: Dull to vitreous; Crystal structure: Coarse-grained, foliated texture with distinct 'gneissic banding'; Cleavage: Poor, but breaks along foliated planes.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded appearance with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: Dull to vitreous; Crystal structure: Coarse-grained, foliated texture with distinct 'gneissic banding'; Cleavage: Poor, but breaks along foliated planes.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks. It undergoes extreme heat and pressure, typically during mountain-building events (orogenies), dating from the Precambrian to recent geological eras.
Uses & applications
Commonly used in construction as crushed stone for road bases and concrete aggregate. High-quality specimens with attractive banding are used for architectural facings, flooring, countertops, and gravestones.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, dated at 4.03 billion years old. The term 'gneiss' is derived from an old German mining term meaning 'sparkling' or 'to strike'.
Field identification & locations
Identify by looking for distinct mineral segregation into light and dark bands (foliation). It is commonly found in the cores of mountain ranges and ancient continental shields. For collectors, look for 'Augen Gneiss' which contains eye-shaped mineral grains.
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