metamorphic
Gneiss Pebble
Gneiss
Hardness: 6-7 on Mohs scale. Color: Banded pattern featuring salmon-pink (alkali feldspar), greyish-white (quartz), and dark greenish-black (biotite/hornblende). Luster: Vitreous to dull. Texture: Medium to coarse-grained with distinct foliation (gneissic banding).
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Banded pattern featuring salmon-pink (alkali feldspar), greyish-white (quartz), and dark greenish-black (biotite/hornblende). Luster: Vitreous to dull. Texture: Medium to coarse-grained with distinct foliation (gneissic banding).
Formation & geological history
Formed by high-grade regional metamorphism of igneous rock (like granite) or sedimentary rock under extreme heat and pressure deep within Earth's crust. Subsequent erosion and water action smoothed it into a pebble.
Uses & applications
Crushed stone for construction, road building, decorative landscaping river rock, and educational specimens.
Geological facts
The alternating dark and light bands are created as minerals segregate under intense directional pressure. Gneiss includes some of the oldest exposed rocks on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old.
Field identification & locations
Identify in the field by looking for distinct, non-continuous alternating bands of light-colored quartz/feldspar and dark ferromagnesian minerals. Commonly found in riverbeds, glacial deposits, and mountain ranges composed of metamorphic terranes.
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