metamorphic
Granite Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Mottled black, pink, and white/grey; Luster: Dull to vitreous when wet; Structure: Foliated or banded with interlocking mineral grains; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Mottled black, pink, and white/grey
- Luster
- Dull to vitreous when wet
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Mottled black, pink, and white/grey; Luster: Dull to vitreous when wet; Structure: Foliated or banded with interlocking mineral grains; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade regional metamorphism of a granitic protolith under intense heat and pressure, causing segregation of mafic and felsic minerals.
Uses & applications
Used primarily in construction for crushed stone or decorative landscaping. High-quality specimens are used for countertops and architectural cladding.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissose banding' or visible segregation of dark (biotite/hornblende) and light (quartz/feldspar) minerals. Found in crystalline basement terrains and mountainous regions.
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