metamorphic
River Cobble (Salt and Pepper Gneiss)
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated texture (gneissic banding); Cleavage: None (tends to fracture across bands); Specific gravity: 2.6–2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, and black
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated texture (gneissic banding); Cleavage: None (tends to fracture across bands); Specific gravity: 2.6–2.9.
Formation & geological history
Formed from the high-pressure and high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths). This specific specimen shows 'rounding,' indicating it was transported and eroded by river water over thousands of years.
Uses & applications
Used primarily in construction as crushed stone for road fill or as decorative landscaping stones. Highly polished gneiss is sometimes used for floor tiles or countertops.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The term 'gneiss' comes from an old German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify in the field by looking for distinct alternating bands of light (quartz/feldspar) and dark (biotite/amphibole) minerals. Commonly found in mountainous regions or riverbeds downstream from metamorphic mountain belts. Collectors look for unique banding patterns.
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