metamorphic
Gneiss
Gneiss (Metamorphic rock of variable composition, typically SiO2 rich)
Hardness: 6-7 on Mohs scale; Color: Characteristic 'salt and pepper' look with bands of white/pink (feldspar/quartz) and black/dark green (biotite/amphibole); Luster: Dull to pearly; Crystal structure: Coarse-grained with a foliated, banded texture known as gneissose banding;…
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to pearly
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: Characteristic 'salt and pepper' look with bands of white/pink (feldspar/quartz) and black/dark green (biotite/amphibole); Luster: Dull to pearly; Crystal structure: Coarse-grained with a foliated, banded texture known as gneissose banding; Cleavage: Poorly defined in the rock mass, though individual minerals have their own.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone) subjected to extreme heat and pressure. It is often found in the roots of mountain ranges and represents some of the oldest crustal exposures on Earth, frequently dating back hundreds of millions or billions of years.
Uses & applications
Used primarily as a construction material (crushed stone for road beds), architectural stone for flooring or countertops, and as ornamental landscaping stone. It is highly durable and resistant to weathering.
Geological facts
Gneiss is the source of the Acasta Gneiss in Canada, which is one of the oldest known intact crustal rocks on Earth, dated at approximately 4.03 billion years old. The bands form because different minerals segregate under intense pressure and temperature without completely melting.
Field identification & locations
Identify in the field by looking for distinct light and dark parallel banding (foliation) where the bands are thicker than those found in schist. It doesn't split easily into thin sheets like slate. Commonly found in metamorphic terrains such as the Appalachian Mountains, the Alps, or the Canadian Shield.
More like this
