metamorphic
Gneiss (Banded Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, white, and tan bands; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, and tan bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, and tan bands; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor; 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.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for flooring or countertops, and as decorative landscaping stone.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal fragments on Earth, dated at approximately 4.03 billion years old. The distinctive banding is called 'gneissic foliation' and is caused by the segregation of minerals into light and dark layers.
Field identification & locations
Look for alternating bands of light-colored minerals (like quartz and feldspar) and dark-colored minerals (like biotite or hornblende). It is commonly found in the roots of mountain ranges and in ancient continental shields.
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