metamorphic
Granite Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs); Color: Pink (Orthoclase), Grey/White (Quartz/Plagioclase), Black (Biotite/Hornblende); Luster: Dull to Vitreous; Structure: Foliated with distinct banding or streaks; Cleavage: None (in bulk rock); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to Vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Pink (Orthoclase), Grey/White (Quartz/Plagioclase), Black (Biotite/Hornblende); Luster: Dull to Vitreous; Structure: Foliated with distinct banding or streaks; Cleavage: None (in bulk rock); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by the high-pressure, high-temperature metamorphism of granite or sedimentary rocks. This specimen likely dates from the Precambrian or Paleozoic eras and formed deep within the Earth's crust during mountain-building events (orogenies).
Uses & applications
Used primarily in construction as crushed stone or dimensions stone (countertops/monuments). Because of its hardness and resistance to weathering, it is ideal for landscaping and architectural facades.
Geological facts
Gneiss contains some of the oldest known rock samples on Earth, such as the Acasta Gneiss in Canada, which is roughly 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'glimmer'.
Field identification & locations
Identify in the field by looking for 'gneissose banding' where dark and light minerals have separated into distinct layers or lenses. It is commonly found in the roots of eroded mountain ranges and glacial till across North America and Scandinavia.
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