metamorphic
Banded Gneiss
Gneiss (Metamorphic high-grade, predominantly SiO2, Al2O3, K2O, Na2O)
Hardness: 6-7 (Mohs scale); Color: Gray, tan, white, and dark charcoal bands; Luster: Dull to slightly vitreous; Crystal Structure: Foliated (gneissose banding); Cleavage: None, but splits along bands; Specific Gravity: 2.6 - 2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, tan, white, and dark charcoal bands
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, tan, white, and dark charcoal bands; Luster: Dull to slightly vitreous; Crystal Structure: Foliated (gneissose banding); Cleavage: None, but splits along bands; Specific Gravity: 2.6 - 2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths (such as shale or granite) under extreme heat and pressure, often during mountain-building events over millions of years.
Uses & applications
Used primarily as decorative stone in landscaping, crushed stone for road construction, building veneer, and occasionally polished for ornamental bookends or paperweights.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada, which is roughly 4.03 billion years old. The name comes from the Middle High German verb 'gneist' meaning 'to spark'.
Field identification & locations
Identified in the field by prominent compositional banding where light and dark minerals are segregated. It is commonly found in the cores of mountain ranges and eroded crystalline shields. Collectors look for unique swirling patterns or 'augens' (eye-shaped mineral grains).
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