metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale. Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands. Luster: Dull to vitreous. Crystal structure: Foliated, medium to coarse-grained. Cleavage: Poor, though it splits along bands.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands. Luster: Dull to vitreous. Crystal structure: Foliated, medium to coarse-grained. Cleavage: Poor, though it splits along bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (granite) or sedimentary (shale/sandstone) rocks. Subjected to intense heat and pressure, causing minerals to segregate into bands or layers.
Uses & applications
Used primarily as crushed stone for road construction and landscaping. Denser varieties are used for architectural facing, floor tiles, and gravestones.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The banding is specifically called 'gneissose banding'.
Field identification & locations
Identify by looking for distinct layers of different minerals (foliation) that are not easily crumbled. Commonly found in the cores of mountain ranges and glacial till. Collectors value unique 'eye' or 'augen' structures.
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