metamorphic
Granite Gneiss
Gneiss (Polymetamorphic Quartz-Feldspar-Mica rock)
Hardness: 6-7 on Mohs scale. Color: Banded pink, white, and dark grey/black. Luster: Dull to vitreous. Structure: Foliated with distinct bands (gneissose banding). Cleavage: Poor, typically breaks along foliation planes. Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, white, and dark grey/black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Banded pink, white, and dark grey/black. Luster: Dull to vitreous. Structure: Foliated with distinct bands (gneissose banding). Cleavage: Poor, typically breaks along foliation planes. Specific Gravity: 2.6-2.9.
Formation & geological history
Formed from the high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. Subjected to intense heat and pressure, causing minerals to recrystallize and segregate into bands. Often Precambrian in age.
Uses & applications
Used widely in construction for dimension stone, flooring, gravestones, and road aggregate. Highly valued as countertops and ornamental stone due to its durability and aesthetic banding.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The word 'gneiss' comes from an Old High German verb meaning 'to spark' because the rock glitters.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—alternating layers of light (felsic) and dark (mafic) minerals. Common in the roots of mountain ranges and crystalline shields. Collectors look for sharp contrast between bands or garnet inclusions.
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