metamorphic
Banded Gneiss and Siltstone River Stones
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Grey, black, white, and tan bands; Luster: Dull to slightly vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, black, white, and tan bands
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, black, white, and tan bands; Luster: Dull to slightly vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-pressure, high-temperature metamorphism of pre-existing igneous or sedimentary rocks. The banding patterns results from the segregation of mineral phases during recrystallization. These specific samples have been rounded and smoothed by fluvial or coastal erosion.
Uses & applications
Used in landscaping, as aquarium decor, for rock painting, and as bulk aggregate in construction. High-quality pieces are used for architectural surfacing.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens dated to over 4 billion years old. The banding is a result of extreme tectonic forces that stretch and compress the rock layer by layer.
Field identification & locations
Identify in the field by looking for distinct light and dark parallel mineral bands (foliation) and a crystalline texture. Very common in glacial till, riverbeds, and mountainous regions like the Appalachians or the Alps.
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