metamorphic
Banded Gneiss
Metamorphic Gneiss (High-grade regional metamorphism specimen)
Hardness: 6-7 (Mohs); Color: Alternating bands of dark green/grey (amphiboles/biotite) and light pink/cream (feldspar/quartz); Luster: Sub-vitreous to dull; Texture: Coarse-grained foliated (gneissic banding).
- Hardness
- 6-7 (Mohs)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating bands of dark green/grey (amphiboles/biotite) and light pink/cream (feldspar/quartz); Luster: Sub-vitreous to dull; Texture: Coarse-grained foliated (gneissic banding).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. The banding occurs due to the segregation of minerals during the metamorphic process, often dating back to the Proterozoic or Archean eras.
Uses & applications
Commonly used as crushed stone for road construction, building stone, architectural facing, and sometimes as polished decorative river stones or pocket stones.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The separation into light and dark layers is called 'gneissic banding'.
Field identification & locations
Identify by the distinct parallel layers of varying mineral composition. It can be found in the roots of mountain ranges or in glacial tills. Collectors look for sharp contrast in the bands and smooth, water-worn shapes.
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