metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Gray, black, and white banded appearance; Luster: Dull to vitreous; Crystal Structure: Foliated with distinct bands (gneissic banding); Cleavage: Generally poor, breaks into irregular fragments; Specific Gravity: 2.6–2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, black, and white banded appearance
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, black, and white banded appearance; Luster: Dull to vitreous; Crystal Structure: Foliated with distinct bands (gneissic banding); Cleavage: Generally poor, breaks into irregular fragments; 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 (like shale). Occurs under extreme temperature and pressure deep within the Earth's crust, often during mountain-building events (orogenies). Many specimens date back to the Precambrian era.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality slabs are used for countertops, flooring, and decorative building stone similar to granite. It is also common in landscaping.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German word meaning 'spark' or 'to glow' because of the way the minerals occasionally glisten.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light minerals (quartz, feldspar) and dark minerals (biotite, amphibole). It lacks the fine splitting (slaty cleavage) of schist. Frequently found in mountainous regions or ancient continental shields.
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