metamorphic
Gneiss (foliated metamorphic rock)
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Luster: dull to vitreous; Crystal structure: Coarse-grained with visible foliated bands; Cleavage: poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded gray, white, and black
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Luster: dull to vitreous; Crystal structure: Coarse-grained with visible foliated bands; Cleavage: poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. This specimen exhibits water-worn rounding, suggesting it was found in a river or coastal environment.
Uses & applications
Crushed stone for road construction, building stone, decorative landscaping, and sometimes as dimension stone for countertops and flooring.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The banding is caused by the segregation of light minerals (quartz, feldspar) and dark minerals (mica, hornblende).
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding' or alternating layers of different minerals. It is common in the roots of mountain ranges and ancient continental shields. Collectors look for unique color patterns and mineral inclusions.
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