metamorphic
Banded Gneiss
Gneiss (High-grade metamorphic rock typically composed of Quartz, Feldspar, and Mica)
Hardness: 6-7 (Mohs); Color: Alternating light (felsic) and dark (mafic) bands in grey, white, and tan; Luster: Vitreous to dull; Structure: Foliated with distinct banding; Cleavage: Generally poor, breaks along foliations; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs)
- Luster
- Vitreous to dull
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 (Mohs); Color: Alternating light (felsic) and dark (mafic) bands in grey, white, and tan; Luster: Vitreous to dull; Structure: Foliated with distinct banding; Cleavage: Generally poor, breaks along foliations; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, often during mountain-building events (orogenies). Ages can range from hundreds of millions to billions of years (Precambrian).
Uses & applications
Used primarily in construction as crushed stone or 'rip-rap,' and specifically for dimension stone in countertops, monuments, and decorative floor tiles due to its durability and aesthetic banding.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal fragments on Earth, dated to approximately 4 billion years old. The banding is the result of 'metamorphic differentiation' where minerals segregate into layers during intense pressure.
Field identification & locations
Identify by 'gneissic banding' (coarse layers of different minerals). Commonly found in the crystalline cores of mountain ranges and stable continental cratons like the Canadian Shield or the Appalachians. Collectors look for unique swirling patterns or 'augens' (eye-shaped crystals).
More like this
