metamorphic
Granitic Gneiss
Gneiss (high-grade metamorphic rock primarily of quartz, feldspar, and mica)
Hardness: 6-7 on Mohs scale; Color: Pink, grey, and black mottled patterns; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with foliated banding or 'salt and pepper' appearance; Cleavage: Poor, but exhibits distinct layering.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, grey, and black mottled patterns
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, grey, and black mottled patterns; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with foliated banding or 'salt and pepper' appearance; Cleavage: Poor, but exhibits distinct layering.
Formation & geological history
Formed through the high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. The distinctive banding occurs under intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Used widely in construction for building stone, pavers, and countertops. It is also crushed into aggregate for road beds and railroad ballast.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. Because of its hardness, it resists weathering exceptionally well.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers. It is ubiquitous in the roots of mountain ranges and glaciated regions. Collectors value specimens with high-contrast banding or large garnet inclusions.
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