metamorphic
Banded Gneiss (River Cobble)
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Alternating white, gray, and dark bands; Luster: Dull to pearly; Crystal structure: Coarse-grained phaneritic; Cleavage: Poor, but exhibits foliation banding; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating white, gray, and dark bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating white, gray, and dark bands; Luster: Dull to pearly; Crystal structure: Coarse-grained phaneritic; Cleavage: Poor, but exhibits foliation banding; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed under high-temperature and high-pressure conditions (regional metamorphism) deep in the Earth's crust. Most specimens date back hundreds of millions to billions of years. This specific piece has been rounded by fluvial (river) or glacial erosion.
Uses & applications
Commonly used as crushed stone in construction, road aggregates, and decorative landscaping. High-quality slabs are used for countertops and grave markers. Small pebbles are popular as 'worry stones' or for aquarium decor.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens in Canada dating back over 4 billion years. The distinct banding is known as 'gneissic banding' caused by the segregation of light and dark minerals during intense pressure.
Field identification & locations
Identified in the field by its distinct parallel layers or bands of color that do not split easily like slate. Commonly found in mountainous regions or in riverbeds downstream from metamorphic outcrops. Collectors should look for high contrast between bands.
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