metamorphic
Gneiss (specifically a Granitic Gneiss)
Gneiss (typically composed of Quartz, Feldspar, and Mica/Amphiboles)
Hardness: 6-7 on Mohs scale; Color: Banded pink, gray, black, and white; Luster: Vitreous to dull; Crystal structure: Course-grained and foliated (banded); Cleavage: Poor (tends to break across bands); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, gray, black, and white
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pink, gray, black, and white; Luster: Vitreous to dull; Crystal structure: Course-grained and foliated (banded); Cleavage: Poor (tends to break across bands); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure. It is often Archean or Proterozoic in age (billions of years old) and found in the crystalline basement of continents.
Uses & applications
Used primarily as crushed stone for road construction and railroad ballast, or as dimension stone for building facades, countertops, and flooring due to its durability and aesthetic banding.
Geological facts
Gneiss is some of the oldest rock on Earth, such as the Acasta Gneiss in Canada, which is dated to approximately 4.03 billion years old. The term 'Gneiss' comes from a Middle High German verb meaning to 'sparkle' or 'glint'.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct layers of light colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). This specific specimen shows rounding from water erosion, likely a river cobble or glacial erratic.
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