metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs); Color: Gray or white with dark bands of black or green; Luster: Dull to Vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poorly developed, usually breaks along foliation planes; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs)
- Color
- Gray or white with dark bands of black or green
- Luster
- Dull to Vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Gray or white with dark bands of black or green; Luster: Dull to Vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poorly developed, usually breaks along foliation planes; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). Exposed to intense heat and pressure, causing minerals to segregate into distinct bands known as gneissic banding.
Uses & applications
Used primarily as a crushed stone in road construction, concrete production, and railway ballast. High-quality specimens are used for countertops, floor tiles, and gravestones.
Geological facts
Gneiss is one of the oldest types of rock on Earth; the Acasta Gneiss in Canada is dated to approximately 4 billion years old. The term comes from an old German verb 'gneist' meaning 'to spark'.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light minerals (quartz, feldspar) and dark minerals (biotite, hornblende). It is commonly found in the roots of mountain ranges and ancient continental shields.
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