metamorphic
Banded Gneiss River Pebble
Gneiss
Hardness: 6-7 Mohs scale. Color: Alternating light gray/cream and dark gray bands. Luster: Dull to vitreous. Structure: Medium to coarse-grained foliated structure with distinct banding (gneissic foliation). Specific gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs scale
- Color
- Alternating light gray/cream and dark gray bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs scale. Color: Alternating light gray/cream and dark gray bands. Luster: Dull to vitreous. Structure: Medium to coarse-grained foliated structure with distinct banding (gneissic foliation). Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme heat and pressure, causing segregation of mineral grains into distinct light and dark bands. Subsequently rounded by fluvial (river) erosion.
Uses & applications
Commonly used as decorative garden stones, landscaping river rocks, and crushed stone in construction. Decorative specimens are collected as natural display stones or paperweights.
Geological facts
Gneiss is one of the oldest rock types found on Earth, with some specimens like the Acasta Gneiss in Canada dating back over 4 billion years. The prominent banding is caused by directional pressure deep within the Earth's crust.
Field identification & locations
Easily identified by its smooth, water-worn, oval shape combined with distinct parallel zebra-like banding of light quartz/feldspar and dark biotite/amphibole minerals. Commonly found along riverbeds, beaches, and glacial deposits globally.
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