metamorphic
Banded Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Gray, tan, white, and dark brown/black banding; Luster: Dull to sub-vitreous; Texture: Coarse-grained with distinct foliation (gneissose banding); Cleavage: Poor, but breaks along grain boundaries.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, white, and dark brown/black banding
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, white, and dark brown/black banding; Luster: Dull to sub-vitreous; Texture: Coarse-grained with distinct foliation (gneissose banding); Cleavage: Poor, but breaks along grain boundaries.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. The distinct bands result from 'metamorphic differentiation' where minerals segregate into layers. These rocks often date back to the Precambrian era.
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and railroad ballast. Exceptionally beautiful specimens are used as dimension stone for countertops, flooring, and cemetery monuments.
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'glitter'.
Field identification & locations
Identify in the field by looking for alternating light-colored (felsic) and dark-colored (mafic) bands that are generally thicker than the laminations in schist. Commonly found in the cores of mountain ranges and ancient continental shields. Collectors look for tight folds or 'ptygmatic' veins within the banding.
