metamorphic
Gneiss (Banded Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure, often at convergent plate boundaries. Gneiss can be billions of years old.
Uses & applications
Used as a decorative stone in architecture, countertops, flooring, and as crushed stone or 'road metal' in construction and landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from the Middle High German word 'gneist', meaning 'to spark'.
Field identification & locations
Identify by looking for distinct layers or bands of color that are not easily split (unlike schist). Commonly found in the cores of mountain ranges and ancient continental shields.
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