metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 Mohs; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Crystal structure: Foliated with coarse-grained textures; Cleavage: Poorly developed along foliation; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Luster
- Dull to vitreous
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 Mohs; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Crystal structure: Foliated with coarse-grained textures; Cleavage: Poorly developed along foliation; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. The process, known as 'gneissic banding,' involves the segregation of minerals. Many specimens date from the Precambrian era (over 541 million years old).
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and ballast. High-quality slabs are used for countertops, flooring, and facing stones in buildings. It is also a popular specimen for educational geology kits.
Geological facts
Gneiss includes some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding'—alternating layers of different mineral compositions. It is commonly found in the cores of mountain ranges and eroded shield areas like the Canadian Shield or the Appalachian Mountains. Collectors should look for sharp color contrasts and unique folding patterns.
More like this
