metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: White, gray, and black banding; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, foliation (gneissic banding); Cleavage: Poor, splits along bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- White, gray, and black banding
- 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
Hardness: 6-7 (Mohs scale); Color: White, gray, and black banding; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, foliation (gneissic banding); Cleavage: Poor, splits along bands; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (protoliths) under intense heat and pressure deep within the earth's crust, often during mountain-building events (orogenies).
Uses & applications
Commonly used in construction as crushed stone for road bases, building stone for cladding and flooring, and occasionally as ornamental slabs for countertops.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The banding is caused by the segregation of light minerals like quartz/feldspar and dark minerals like biotite/hornblende.
Field identification & locations
Identify by distinct 'salt and pepper' banding or stripes that represent mineral segregation. It is found globally in the cores of mountain ranges and ancient continental shields. Collectors look for unique folding patterns in the bands.
