metamorphic
Gneiss (Cobble)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, white, pink, and black bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor, usually breaks along uneven surfaces.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, pink, and black bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, pink, and black bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (granite) or sedimentary (shale/mudstone) rocks. This specimen has been rounded by fluvial (river) or glacial transport processes.
Uses & applications
Used as crushed stone for road construction, decorative landscaping stones, and high-quality slabs for countertops (often sold as 'granite' in the trade).
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The distinct stripes are formed because different minerals migrate into layers under intense heat and pressure.
Field identification & locations
Identify in the field by looking for 'banding' where light minerals (quartz/feldspar) and dark minerals (biotite/hornblende) are separated into stripes. Commonly found in mountainous regions or glacial till.
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