metamorphic
Gneiss
Gneiss
Hardness of 6-7 on the Mohs scale; distinctive banded or foliated appearance featuring alternating lighter bands of quartz and feldspar and darker bands of biotite and amphibole; dull to subvitreous luster; specific gravity around 2.6-2.9.
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Physical properties
Hardness of 6-7 on the Mohs scale; distinctive banded or foliated appearance featuring alternating lighter bands of quartz and feldspar and darker bands of biotite and amphibole; dull to subvitreous luster; specific gravity around 2.6-2.9.
Formation & geological history
Formed by high-temperature and high-pressure metamorphic processes acting on pre-existing igneous or sedimentary rocks (such as granite or shale) deep within the Earth's crust, typically during continental collisions.
Uses & applications
Used extensively in construction as crushed stone for road aggregates, building stone, and dimension stone for paving or facing.
Geological facts
Gneiss contains some of the oldest rocks found on Earth, with certain formations dating back over 4 billion years.
Field identification & locations
Easily identified in the field by its characteristic 'gneissic banding' (zebra-like stripes of light and dark minerals). Commonly found in ancient shields and core areas of mountain ranges.
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