metamorphic
Gneiss (Granitic Gneiss)
Gneiss
Hardness: 6-7 (Mohs scale); Color: Gray, pink, or white with dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct gneissic banding; Cleavage: Generally poor, breaks along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, pink, or white with dark bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, pink, or white with dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct gneissic banding; Cleavage: Generally poor, breaks along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under high pressure and temperature. Mostly Precambrian or Paleozoic in age.
Uses & applications
Used as a decorative building stone, crushed stone for road construction, pavement, and landscaping. Occasionally used for curbstones and grave markers.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old.
Field identification & locations
Identify by looking for 'gneissic banding' where minerals are segregated into visible layers. Common in continental shields and mountain cores. Use a hand lens to see interlocking crystals.
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