metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs); Color: Alternating dark and light bands of gray, black, and white; Luster: Dull to vitreous; Texture: Coarse-grained with distinct foliation (gneissose banding); Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating dark and light bands of gray, black, and white
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating dark and light bands of gray, black, and white; Luster: Dull to vitreous; Texture: Coarse-grained with distinct foliation (gneissose banding); Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure. Most gneiss is hundreds of millions to billions of years old.
Uses & applications
Used primarily as crushed stone for road construction and railroad ballast, or as dimension stone for building facades, floor tiles, and grave markers. High-quality specimens are used for countertops.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rock formations on Earth, dating back approximately 4 billion years. The banding is caused by the segregation of minerals during intense pressure.
Field identification & locations
Identify by looking for 'gneissose banding'—alternating layers of light (quartz, feldspar) and dark (biotite, hornblende) minerals. Found globally in the roots of mountain ranges and ancient continental shields.
