metamorphic
Gneiss
Gneiss
Hardness: 6-7 Mohs; Color: Gray, pink, or brownish with light and dark bands; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poorly developed along bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Gray, pink, or brownish with light and dark bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pink, or brownish with light and dark bands; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poorly developed along bands; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like schist or shale) under intense heat and pressure deep within the Earth's crust.
Uses & applications
Used primarily as dimension stone for building facades, paving, and kitchen countertops; also used as crushed stone in construction and landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The name comes from the German word 'Gneist', meaning spark.
Field identification & locations
Identify in the field by looking for 'gneissic banding', where minerals are segregated into distinct light (quartz/feldspar) and dark (biotite/amphibole) layers. Common in mountainous regions and continental shields like the Canadian Shield.
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