metamorphic
Gneiss
High-grade regional metamorphic rock (Foliated Gneiss)
Foliated texture with distinct color banding (gneissose banding), hardness of 6-7 on Mohs scale, color ranges from grey to bluish-green in this specimen, dull to pearly luster, coarse-grained, specific gravity 2.6-2.9.
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Physical properties
Foliated texture with distinct color banding (gneissose banding), hardness of 6-7 on Mohs scale, color ranges from grey to bluish-green in this specimen, dull to pearly luster, coarse-grained, specific gravity 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, typically during mountain-building events (orogenies).
Uses & applications
Used in construction as crushed stone for road bases and concrete, as dimension stone for building facades, counter tops, and as decorative river pebbles or landscaping stones.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The banding is caused by the segregation of mafic and felsic minerals during intense pressure.
Field identification & locations
Identified by 'gneissose banding' where minerals are arranged in alternating light and dark layers. Commonly found in the cores of mountain ranges and eroded crystalline shields. For collectors, it is a classic example of high-grade metamorphism.
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