metamorphic
Gneiss (specifically Banded Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black 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
- Pink, gray, white, and black bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black 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 rocks (like granite) or sedimentary rocks under intense heat and pressure. Most gneiss is hundreds of millions to billions of years old.
Uses & applications
Used primarily as decorative stone, crushed stone for road construction, and occasionally for countertops and floor tiles when polished.
Geological facts
Gneiss often contains some of the oldest minerals on Earth, such as zircons. It is famous for the 'gneissose banding' where minerals segregate into distinct dark and light layers.
Field identification & locations
Identify by looking for distinct alternating bands of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). It is commonly found in the roots of mountain ranges and glacial deposits.
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