metamorphic
Granite Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade regional metamorphism of granite or sedimentary protoliths. This specimen shows significant heat and pressure transformation, typically occurring deep within the Earth's crust during mountain-building events (orogeny).
Uses & applications
Used primarily in construction as crushed stone for road bases, architectural building stone, and decorative landscaping. High-quality slabs are used for countertops and flooring.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated to approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark'.
Field identification & locations
Identify in the field by looking for 'salt and pepper' appearances with alternating layers of light-colored minerals (feldspar/quartz) and dark-colored minerals (biotite/hornblende). It is commonly found in the roots of eroded mountain ranges and Precambrian shield areas.
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