metamorphic
Gneiss
Gneiss
Hardness: 6.5-7 Mohs; Color: Gray, pink, white, or black; Luster: Dull to vitreous; Crystal structure: Crystalline with characteristic foliation (gneissic banding); Cleavage: Poorly developed in large scale but mica layers exhibit basal cleavage; Specific gravity: 2.6-2.9.
- Hardness
- 6
- Color
- Gray, pink, white, or black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Gray, pink, white, or black; Luster: Dull to vitreous; Crystal structure: Crystalline with characteristic foliation (gneissic banding); Cleavage: Poorly developed in large scale but mica layers exhibit basal cleavage; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. It is often found in the cores of mountain ranges and ancient shields like the Canadian Shield.
Uses & applications
Used primarily as crushed stone for road construction and railroad ballast; higher-quality specimens are used as architectural stone, countertops, and decorative landscaping rock.
Geological facts
Gneiss is one of the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The term comes from the Middle High German word 'gneist', meaning to spark.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—distinct light and dark layered mineral stripes. Common in glacial till and mountainous areas. Collectors look for unique banding patterns or accessory minerals like garnets.
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