metamorphic
Gneiss
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black banded; Luster: Vitreous to dull; Crystal structure: Coarse-grained phaneritic with distinct foliation; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Gray, pink, white, and black banded
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black banded; Luster: Vitreous to dull; Crystal structure: Coarse-grained phaneritic with distinct foliation; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. It often dates back hundreds of millions to billions of years.
Uses & applications
Used primarily as crushed stone for road construction and railroad ballast, or polished for architectural facings, flooring, and cemetery monuments.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada estimated to be 4.03 billion years old. The banding is caused by the segregation of light and dark minerals during metamorphism.
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding', where minerals like quartz/feldspar and biotite/hornblende separate into alternating light and dark layers. Common in mountainous regions or ancient continental shields.
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