metamorphic
Gneiss
Gneiss (Metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: Banded gray, white, tan, and dark green/black, Luster: Dull to sub-vitreous, Crystal structure: Coarse-grained with distinct foliation (gneissic banding)
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Banded gray, white, tan, and dark green/black, Luster: Dull to sub-vitreous, Crystal structure: Coarse-grained with distinct foliation (gneissic banding)
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. Typically found in ancient metamorphic terranes and mountain belts dating from the Precambrian to Cenozoic eras.
Uses & applications
Used as crushed stone for road construction, as dimension stone for architectural facades, floor tiles, and grave markers. High durability makes it suitable for heavy construction.
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. It is distinguished by the segregation of light and dark minerals into alternating layers.
Field identification & locations
Identify in the field by looking for 'gneissic banding' where minerals are separated into distinct layers. Common in the Piedmont region of the US, the Alps, and high-mountain ranges. Excellent for collectors interested in structural geology.
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