metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Luster: dull to vitreous; Crystal structure: coarse-grained crystalline; Cleavage: none (shows foliation/banding); 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 crystalline; Cleavage: none (shows foliation/banding); Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (protoliths) under intense heat and pressure, often at depths of 10-20km. This specimen displays characteristic 'gneissic banding'.
Uses & applications
Commonly used as crushed stone in construction, road aggregates, and occasionally as decorative building stone or countertops (often sold commercially as 'granite').
Geological facts
Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is caused by the segregation of light minerals like quartz and feldspar from dark minerals like biotite or hornblende.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating light and dark layers. It is ubiquitous in the roots of mountain ranges and ancient continental shields. Found worldwide in areas like the Appalachian Mountains, Scandinavia, and the Canadian Shield.
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