metamorphic
Granitic Gneiss
Gneiss (Metamorphic Rock)
Hardiness: 6-7 on Mohs scale; Color: Gray, pinkish-white, and dark gray bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, typically breaks along foliation planes; Specific gravity: 2.6 - 2.9
- Color
- Gray, pinkish-white, and dark gray bands
- Luster
- Dull to vitreous
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Physical properties
Hardiness: 6-7 on Mohs scale; Color: Gray, pinkish-white, and dark gray bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, typically breaks along foliation planes; Specific gravity: 2.6 - 2.9
Formation & geological history
Formed through the high-grade regional metamorphism of igneous granite or sedimentary protoliths under extreme pressure and temperature. This specimen appears as a river-worn or glacially rounded cobble, smoothed over thousands of years.
Uses & applications
Used primarily in construction as crushed stone for road bases or as architectural facing and flooring. High-quality slabs are used for countertops (often sold commercially as 'granite').
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term originates from the Middle High German word 'gneist', meaning 'spark'.
Field identification & locations
Identify by 'gneissic banding'—the visible separation of light minerals (quartz, feldspar) and dark minerals (mica, hornblende). Found globally in deeply eroded mountain belts and shields. Collectors should look for distinct mineral layering and lack of visible fossils.
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