metamorphic
Gneiss
Gneiss
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Structure: Foliated with distinct composition banding; Cleavage: Poorly developed along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Gray, pink, white, and black banded
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Structure: Foliated with distinct composition banding; Cleavage: Poorly developed along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of igneous (orthogneiss) or sedimentary (paragneiss) rocks under extreme heat and pressure. It is often found in the cores of mountain belts and ancient crystalline shields.
Uses & applications
Used as crushed stone for road construction and aggregate; high-quality specimens are used for countertops, floor tiles, and as ornamental building facing.
Geological facts
Gneiss is one of the oldest rocks on Earth; the Acasta Gneiss in Canada is dated to approximately 4 billion years old. The term comes from an old German word meaning 'sparkling'.
Field identification & locations
Identify by looking for 'gneissose banding', where light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, hornblende) separate into distinct layers. Found in mountainous or glaciated regions like the Alps, Appalachians, and the Canadian Shield.
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