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; Structure: Foliated with coarse-grained Gneissose banding; Cleavage: Poorly defined due to mineral interference; 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; Structure: Foliated with coarse-grained Gneissose banding; Cleavage: Poorly defined due to mineral interference; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (orthogneiss) or sedimentary (paragneiss) rocks. It forms under intense pressure and temperatures exceeding 600°C, often during mountain-building events (orogenies). It is found in ancient continental shields like the Canadian Shield, dating back over 3.8 billion years.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality slabs are used for flooring, building facades, and kitchen countertops (often sold as 'granite'). It is also collected as a representative specimen of deep-crustal geological processes.
Geological facts
The Acasta Gneiss in the Northwest Territories of Canada is one of the oldest known crustal rock formations on Earth, aged at approximately 4.03 billion years. Gneiss is distinct from schist because its minerals are segregated into visible layers rather than being evenly distributed.
Field identification & locations
Identify in the field by looking for 'Gneissose banding'—distinct light and dark layers that are often contorted or folded. Unlike schist, it does not split easily along planes. Common in metamorphic terrains like the Appalachians, Alps, and Himalayas. Collectors look for 'Augen Gneiss' which contains eye-shaped mineral grains.
More like this
