metamorphic
Gneiss
Gneissic Metamorphic Rock
Hardness: 6-7 on Mohs scale; Color: Distinct alternating bands of light (quartz, feldspar) and dark (biotite, hornblende); Luster: Vitreous to dull; Structure: Gneissic foliation with coarse grains; Cleavage: Poor, due to interlocked grains.
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Distinct alternating bands of light (quartz, feldspar) and dark (biotite, hornblende); Luster: Vitreous to dull; Structure: Gneissic foliation with coarse grains; Cleavage: Poor, due to interlocked grains.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing sedimentary or igneous rocks at deep crustal levels under high pressure and temperature (excess of 600 degrees Celsius). Many specimens are Precambrian in age.
Uses & applications
Used in the construction industry for dimension stone, crushed stone for road bases, architectural facings, and as countertops. Well-banded stones are popular with collectors as decorative river rocks.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The term comes from the German word 'gneist', meaning to spark.
Field identification & locations
Identify in the field by looking for 'gneissose banding', where minerals are segregated into distinct layers. Unlike schist, it does not split easily along planes. Often found in mountainous regions or glacial deposits.
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