metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Features distinct gneissic banding of light and dark minerals. Hardness: 6 to 7 on Mohs scale. Luster: Dull to vitreous. Grain size: Medium to coarse. Mineralogy: Quartz, feldspar, biotite, and amphibole.
- Hardness
- 6 to 7 on Mohs scale
- Luster
- Dull to vitreous
Identified More metamorphic →
Explore Gneiss in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Features distinct gneissic banding of light and dark minerals. Hardness: 6 to 7 on Mohs scale. Luster: Dull to vitreous. Grain size: Medium to coarse. Mineralogy: Quartz, feldspar, biotite, and amphibole.
Formation & geological history
Formed by high-pressure and high-temperature regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone). These specimens are often billions of years old, dating back to the Precambrian era.
Uses & applications
Commonly used as crushed stone for construction, road base, and concrete aggregate. Polished pieces are used for countertops, flooring, and decorative building stone.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding, called foliation, occurs because minerals segregate under intense heat and pressure.
Field identification & locations
Identified by light (felsic) and dark (mafic) parallel bands. It can be found in the core of mountain ranges, glacial till in North America, and stable continental shields. Collectors look for sharp, high-contrast banding.
More like this
