metamorphic
Gneiss (Metamorphosed Pebble)
Gneiss with Quartz/Feldspar banding
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and white; Luster: Dull/earthy to slightly greasy; Crystal structure: Medium-to-coarse grained with distinct banding (gneissic foliation); Cleavage: Distinct between bands; Specific gravity: Approx 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, and white
- Luster
- Dull/earthy to slightly greasy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and white; Luster: Dull/earthy to slightly greasy; Crystal structure: Medium-to-coarse grained with distinct banding (gneissic foliation); Cleavage: Distinct between bands; Specific gravity: Approx 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths. The high pressure and temperature cause recrystallization and segregation of minerals into alternating light and dark layers. This specimen has been significantly water-worn into a rounded pebble.
Uses & applications
Common industrial uses include crushed stone for road construction, railroad ballast, and dimension stone for building facings or landscaping.
Geological facts
Gneiss is one of the most common rocks in the Earth's crust and contains some of the oldest rock formations on the planet, such as the Acasta Gneiss which is over 4 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating bands of different minerals. This rounded specimen is likely from a riverbed or coastal area. Look for its characteristic stripes even when worn smooth.
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