metamorphic
Gneiss (likely Granitic Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Grey, pink, and dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, pink, and dark bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, pink, and dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); 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 high temperature and pressure, typically found in ancient crystalline basements (Precambrian age).
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and sometimes as dimension stone for architectural facings or landscape design.
Geological facts
Gneiss often shows a 'salt and pepper' appearance and contains the same minerals as granite, but they are rearranged into bands. It makes up a large portion of the Earth's lower continental crust.
Field identification & locations
Identify by looking for distinct alternating layers of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). Common in mountainous regions and glacial till in the Northern US and Canada.
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