metamorphic
Gneiss (specifically Biotite Gneiss)
Gneiss (high-grade metamorphic rock primarily composed of Silicates)
Hardness: 6-7 on Mohs scale; Color: Banded grey, black, and white with tan inclusions; Luster: Dull to pearly (due to mica content); Crystal Structure: Granular/Crystalline with foliated banding; Cleavage: Poor (tends to fracture rather than split like slate); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, black, and white with tan inclusions
- Luster
- Dull to pearly (due to mica content)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, black, and white with tan inclusions; Luster: Dull to pearly (due to mica content); Crystal Structure: Granular/Crystalline with foliated banding; Cleavage: Poor (tends to fracture rather than split like slate); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths. It undergoes intense heat and pressure deep within the Earth's crust, causing minerals to recrystallize and segregate into distinct bands. These rocks range from hundreds of millions to billions of years old.
Uses & applications
Used primarily as a crushed stone for road construction, concrete aggregate, and railroad ballast. High-quality slabs are used for architectural flooring, countertops (often sold as 'Granite'), and gravestones.
Geological facts
Gneiss is some of the oldest rock found on Earth, such as the Acasta Gneiss in Canada which is roughly 4.03 billion years old. The term 'Gneiss' comes from an old German verb meaning 'to spark' or 'to gleam' due to the mica content.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers. It is ubiquitous in the roots of mountain ranges and stable continental interiors (shields). Collectors look for specimens with Sharp mineral segregation or garnets.
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