metamorphic
Banded Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray with distinct dark and light mineral bands; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with foliation; Cleavage: Poor (splits along bands); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray with distinct dark and light mineral bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray with distinct dark and light mineral bands; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with foliation; Cleavage: Poor (splits along bands); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under high temperature and pressure. It is often found in the deep cores of mountain ranges and ancient continental shields, dating back hundreds of millions to billions of years.
Uses & applications
Commonly used as a decorative building stone, in flooring, as crushed stone for road construction, and occasionally for countertops and graveyard monuments.
Geological facts
Gneiss is one of the oldest known types of rock on Earth, with some specimens from the Acasta Gneiss complex in Canada dating back over 4 billion years. The banding is caused by 'gneissic foliation' where minerals segregate into layers.
Field identification & locations
Identify by looking for 'gneissic banding'—alternating layers of light minerals (quartz/feldspar) and dark minerals (biotite/amphibole). It is found globally in mountainous and glacial regions. For collectors, look for contrasting colors and sharp layering.
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