metamorphic
Banded Gneiss
Gneiss (quartz-feldspar-biotite metamorphic rock)
Hardness: 6-7 (on Mohs scale); Color: Variegated grey, charcoal, and tan bands; Luster: Dull to vitreous; Crystal Structure: Foliated (layered texture); Cleavage: Usually poor, but breaks along foliations; Specific Gravity: 2.6 - 2.9
- Hardness
- 6-7 (on Mohs scale)
- Color
- Variegated grey, charcoal, and tan bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (on Mohs scale); Color: Variegated grey, charcoal, and tan bands; Luster: Dull to vitreous; Crystal Structure: Foliated (layered texture); Cleavage: Usually poor, but breaks along foliations; Specific Gravity: 2.6 - 2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Under extreme heat and pressure, minerals segregate into aesthetic bands (gneissic banding). Usually dates back to Proterozoic or Archean eras.
Uses & applications
Used primarily as decorative stone, crushed stone for road construction, landscape gravel, and occasionally for lapidary work like paperweights or jewelry.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old. The banding is a result of chemical reactions and structural changes occur in solid state, never fully melting.
Field identification & locations
Identify in the field by distinct alternating light and dark bands (foliation). Common in crystalline basement complexes, glaciated riverbeds, and mountainous regions like the Appalachians or Alps. Collectors look for high-contrast banding and smooth river-worn textures.
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