metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: dull to vitreous; Crystal structure: granular/foliated; Cleavage: poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: dull to vitreous; Crystal structure: granular/foliated; Cleavage: poor; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing sedimentary (paragneiss) or igneous (orthogneiss) rocks under intense heat and pressure. It is often found in the cores of mountain ranges and ancient continental shields (cratons), often dating back millions or billions of years.
Uses & applications
Used as a decorative stone in construction, flooring, gravestones, and as crushed stone for road construction and landscaping.
Geological facts
The Acasta Gneiss in the Northwest Territories, Canada, is one of the oldest known crustal rock formations on Earth, dating back approximately 4.03 billion years.
Field identification & locations
Identify by distinct 'gneissic banding' where minerals are segregated into visible layers. Commonly found in glaciated regions, riverbeds, and mountain outcrops. Collectors look for unique banding patterns or garnet inclusions.
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