metamorphic
Banded Gneiss
Gneiss (Metamorphic rock of variable mineralogy, often quartz-feldspar base)
Hardness: 6-7 on Mohs scale; Color: alternating dark and light bands of gray, white, and tan; Luster: dull to vitreous; Crystal structure: foliated/granular; Cleavage: none, but breaks along bands; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- alternating dark and light bands of gray, white, and tan
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: alternating dark and light bands of gray, white, and tan; Luster: dull to vitreous; Crystal structure: foliated/granular; Cleavage: none, but breaks along bands; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. The intense heat and pressure cause minerals to segregate into distinct bands, a process known as gneissic banding. Most specimens are Proterozoic to Archean in age.
Uses & applications
Used primarily as crushed stone for road construction and aggregate. High-quality specimens are used in architectural stone (countertops, flooring) and as decorative landscape elements or aquarium stones.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens dating back over 4 billion years. The name comes from an Old German word meaning 'sparkling'.
Field identification & locations
Identify in the field by looking for 'gneissic banding' (segregated light and dark layers). It is more durable than schist and does not split as easily. Commonly found in mountain belts and Precambrian shield areas like the Canadian Shield.
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