metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: alternating bands of light (quartz, feldspar) and dark (biotite, amphibole) minerals; Luster: vitreous to dull; Crystal structure: crystalline but with a foliated (gneissic banding) texture; Cleavage: poor, though it may break along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Luster
- vitreous to dull
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 on Mohs scale; Color: alternating bands of light (quartz, feldspar) and dark (biotite, amphibole) minerals; Luster: vitreous to dull; Crystal structure: crystalline but with a foliated (gneissic banding) texture; Cleavage: poor, though it may break along bands; Specific 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 or sandstone) under extreme heat and pressure, typically during mountain-building events (orogeny). Often Precambrian to Paleozoic in age.
Uses & applications
Commonly used as crushed stone in road construction and as a decorative building stone for countertops, flooring, and exterior cladding due to its hardness and attractive banding.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is nearly 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 'gneissic banding', where minerals are segregated into distinct layers. It is found in the deeply eroded cores of mountain ranges and in ancient continental shields. Collectors look for unique ptygmatic folds or rare accessory minerals like garnet.
More like this
