metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded or streaked with gray, pink, tan, and black; Luster: dull to vitreous; Crystal structure: phaneritic/granular with distinct foliation; Density: 2.6-2.9 g/cm³
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded or streaked with gray, pink, tan, and black
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded or streaked with gray, pink, tan, and black; Luster: dull to vitreous; Crystal structure: phaneritic/granular with distinct foliation; Density: 2.6-2.9 g/cm³
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Commonly used as crushed stone for road construction, railway ballast, and as a durable building material (facade stone, pavers, or countertops).
Geological facts
Gneiss represents one of the highest grades of metamorphism. Some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada (approx. 4 billion years old), belong to this category.
Field identification & locations
Identified by 'gneissic banding'—alternating layers of light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, hornblende). It is commonly found in the roots of mountain ranges and ancient continental shields.
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