metamorphic
Gneiss
Foliated Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and pink; Luster: dull to vitreous; Structure: coarse-grained with distinct foliation (banding); SG: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded gray, white, and pink
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and pink; Luster: dull to vitreous; Structure: coarse-grained with distinct foliation (banding); SG: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature, often associated with mountain-building mountain processes (orogeny).
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for architectural facings, flooring, and gravestones. Coarsely banded specimens are sometimes used as landscaping rocks.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated to approximately 4.03 billion years old. The term comes from a Middle High German verb meaning 'to spark' because of its glittery minerals.
Field identification & locations
Identify in the field by looking for 'gneissose banding', where light and dark minerals are segregated into distinct parallel layers. It is commonly found in the cores of mountain ranges and eroded crystalline shields like the Canadian Shield.
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