metamorphic
Foliated Gneiss (River-Worn Pebble)
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Salt-and-pepper appearance (gray, white, and black); Luster: Sub-vitreous to dull; Crystal structure: Coarse-grained with visible 'gneissic banding'; Cleavage: None (breaks irregularly); Specific gravity: 2.6 - 2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Salt-and-pepper appearance (gray, white, and black)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Salt-and-pepper appearance (gray, white, and black); Luster: Sub-vitreous to dull; Crystal structure: Coarse-grained with visible 'gneissic banding'; Cleavage: None (breaks irregularly); 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. Most gneiss specimens date back to the Precambrian era, often exceeding 500 million to several billion years in age.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and occasionally as architectural stone or 'granite' countertops. Small weathered specimens serve as common education tools or decorative garden stones.
Geological facts
Gneiss is the most common rock type in the Earth's lower continental crust. The Acasta Gneiss in Canada is one of the oldest known crustal rocks on Earth, dated at approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for 'foliation'—parallel bands of different mineral colors. This specific specimen has been rounded and smoothed by water action in a river or beach environment. Common in mountain belts and continental shields.
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