metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: typically banded with pink/orange (feldspar), white (quartz), and black/dark gray (biotite/amphibole); Luster: dull to pearly; Crystal structure: coarse-grained with foliation (gneissic banding); Cleavage: poor, tends to break into blocky fragments.
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with pink/orange (feldspar), white (quartz), and black/dark gray (biotite/amphibole); Luster: dull to pearly; Crystal structure: coarse-grained with foliation (gneissic banding); Cleavage: poor, tends to break into blocky fragments.
Formation & geological history
Formed by the high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme temperature and pressure. Found in the cores of mountain ranges and ancient continental shields.
Uses & applications
Used in construction as crushed stone for road bases or as 'granite' dimension stone for countertops, flooring, and decorative cladding. Also significant for geological mapping of tectonic history.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an Old High German word meaning 'to spark' or 'bright'.
Field identification & locations
Identifiable by its 'gneissic banding' where minerals are separated into distinct layers of light and dark. It is commonly found in metamorphic terranes worldwide, including the Appalachian and Himalayan mountains.
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