metamorphic
Gneiss (Metamorphic Pebble)
Gneissic Metamorphic Rock
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black banded/speckled pattern; Luster: Dull to slightly vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, and black banded/speckled pattern
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black banded/speckled pattern; Luster: Dull to slightly vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (such as granite or shale) under intense heat and pressure. This specimen shows water-worn smoothing, typical of river or beach cobbles.
Uses & applications
Commonly used as crushed stone in construction, decorative landscaping, and occasionally as dimension stone for countertops or flooring when in large slabs.
Geological facts
Gneiss exhibits 'gneissic banding', where minerals segregate into light (felsic) and dark (mafic) layers. It is some of the oldest rock found on Earth's crust, with some specimens dating back over 4 billion years.
Field identification & locations
Identify by looking for distinct alternating bands of light and dark minerals. This specific specimen is a 'river rock', found in glacial till, riverbeds, or coastal areas where water has rounded the jagged edges of the original rock.
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