metamorphic
Gneiss (Banded Gneiss)
Gneiss
Hardness: 6-7 (Mohs scale); Color: alternating pink, white, and black/dark grey bands; Luster: dull to pearly; Crystal structure: Coarse-grained foliated (gneissic banding); Cleavage: poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- alternating pink, white, and black/dark grey bands
- Luster
- dull to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: alternating pink, white, and black/dark grey bands; Luster: dull to pearly; Crystal structure: Coarse-grained foliated (gneissic banding); Cleavage: poor; 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 shale) under extreme pressure and temperature. This process allows minerals to recrystallize and segregate into bands. Often Precambrian in age.
Uses & applications
Used primarily as a dimension stone for construction, flooring, decorative facing, and grave markers. Crushed stone for road construction and landscaping.
Geological facts
The word 'Gneiss' comes from a Middle High German verb meaning 'to spark' or 'glisten.' Some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada (approx. 4 billion years old), are gneissic.
Field identification & locations
Identified by distinct planar coloring called 'gneissic banding' or foliation where light and dark minerals are separated. Common in the cores of mountain ranges and eroded crystalline shields (e.g., Canadian Shield, Scandinavia).
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