metamorphic
Gneiss (High-grade Metamorphic Rock)
Gneiss
Hardness: 6-7 Mohs; Color: Salt and pepper appearance, shades of tan, brown, and grey; Luster: Variable, often sparkly due to mica; Structure: Foliated with distinct banding (gneissose banding).
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper appearance, shades of tan, brown, and grey
- Luster
- Variable, often sparkly due to mica
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper appearance, shades of tan, brown, and grey; Luster: Variable, often sparkly due to mica; Structure: Foliated with distinct banding (gneissose banding).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is subjected to intense heat and pressure, often at depths of 10-20 km within the Earth's crust.
Uses & applications
Used primarily as crushed stone in construction, for road base, as decorative landscaping stone, and occasionally as dimension stone for countertops or building facades.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens in the Acasta Gneiss complex of Canada dating back over 4 billion years.
Field identification & locations
Identify in the field by looking for distinct alternating bands of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/hornblende). It is extremely common in the roots of mountain ranges and shield areas globally.
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