metamorphic
Gneiss (Metamorphic Pebble)
Metamorphic Rock (Quartz-Feldspar-Biotite Gneiss)
Hardness: 6-7 on the Mohs scale. Color: Mottled gray, black, pinkish-white, and brown. Luster: Dull to sub-vitreous. Texture: Medium to coarse-grained with characteristic metamorphic foliation or banding of dark biotite/amphibole and light quartz/feldspar.
- Hardness
- 6-7 on the Mohs scale
- Color
- Mottled gray, black, pinkish-white, and brown
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale. Color: Mottled gray, black, pinkish-white, and brown. Luster: Dull to sub-vitreous. Texture: Medium to coarse-grained with characteristic metamorphic foliation or banding of dark biotite/amphibole and light quartz/feldspar.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep within the Earth's crust.
Uses & applications
Mainly used as crushed stone for construction, road base, aggregate, decorative landscaping rock, and for geological collections.
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in northern Canada is approximately 4.0 billion years old, making it one of the oldest known intact crustal rocks.
Field identification & locations
Look for distinct light and dark compositional banding or patchy metamorphic texture in a hard, durable stream or glacial pebble. Highly common in glaciated regions and riverbeds worldwide.
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