metamorphic
Gneiss (River Cobble)
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Banded grey, brown, and tan. Luster: Dull to sub-vitreous. Structure: Foliated with distinct mineral banding (gneissic banding). Cleavage: Poor, usually breaks along irregular surfaces. Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, brown, and tan
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Banded grey, brown, and tan. Luster: Dull to sub-vitreous. Structure: Foliated with distinct mineral banding (gneissic banding). Cleavage: Poor, usually breaks along irregular surfaces. Specific Gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. This specific specimen has been further shaped by fluvial (river) erosion, which smoothed and rounded its edges over thousands of years.
Uses & applications
Commonly used as crushed stone for road construction and railroad ballast. Significant specimens are used for architectural facing, countertops, and garden landscaping.
Geological facts
Gneiss is one of the oldest rock types found on Earth, with some specimens in the Acasta Gneiss complex dating back over 4 billion years. Its name comes from the Middle High German word 'gneist', meaning to spark.
Field identification & locations
Identify in the field by looking for alternating light and dark mineral bands (foliation) and a smooth, rounded 'potato' shape if found near water. Extremely common in Canadian Shield and Appalachian regions and widely available for amateur collectors.
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