metamorphic
Gneiss (High-grade Metamorphic Rock)
Gneiss (Metamorphic silicate rock)
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and black; Luster: Dull to sub-vitreous; Structure: Foliated with distinct banding (gneissose banding); Texture: Coarse-grained; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, and black
- Luster
- Dull to sub-vitreous
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and black; Luster: Dull to sub-vitreous; Structure: Foliated with distinct banding (gneissose banding); Texture: Coarse-grained; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme pressure and temperature conditions deep within the Earth's crust. This specimen shows significant rounding, suggesting a fluvial (river) or glacial transport history.
Uses & applications
Used primarily in construction as crushed stone or 'road metal,' facing stone on buildings, and sometimes as ornamental landscaping stones due to its durability.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is over 4 billion years old. The term 'gneiss' comes from an Old High German word meaning 'to sparkle' or 'to glow.'
Field identification & locations
Identify in the field by looking for 'gneissose banding'—alternating light (quartz/feldspar) and dark (biotite/amphibole) mineral layers. Frequently found in the cores of mountain ranges and stable continental interiors known as shields.
More like this
