metamorphic
Gneiss (Banded Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Dark grey to black with light tan or cream bands; Luster: Dull to slightly vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poorly developed, usually breaks along uneven surfaces.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black with light tan or cream bands
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black with light tan or cream bands; Luster: Dull to slightly vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poorly developed, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. This specimen shows a high degree of recrystallization, common in Precambrian shields and mountain roots.
Uses & applications
Commonly used as crushed stone for road construction, as dimension stone for counter tops or flooring, and as landscaping material. Smaller polished specimens are kept by mineral collectors.
Geological facts
The oldest known rocks on Earth are the Acasta Gneiss in Canada, dated to approximately 4 billion years old. The banding occurs because minerals of different densities and compositions separate under extreme pressure.
Field identification & locations
Identify in the field by looking for alternating light and dark mineral bands (foliation) that appear thicker and more distinct than schist. It is commonly found in the Appalachian Mountains, the Canadian Shield, and the Himalayas.
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