metamorphic
Granitic Gneiss Pebble
Gneiss (felsic metamorphic rock)
Hardness 6-7 on Mohs scale. Mottled gray, tan, pinkish-orange, and black colors. Dull to vitreous luster. Coarse-grained texture showing weak compositional banding (gneissic foliation). Highly durable and water-rounded.
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Physical properties
Hardness 6-7 on Mohs scale. Mottled gray, tan, pinkish-orange, and black colors. Dull to vitreous luster. Coarse-grained texture showing weak compositional banding (gneissic foliation). Highly durable and water-rounded.
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite or sedimentary protoliths under intense heat and pressure deep within the Earth's crust. Subsequently eroded, weathered, and rounded by stream or wave action into a smooth pebble.
Uses & applications
Used primarily as decorative river rock, landscaping material, aggregate for concrete, and as a educational pocket specimen for geology enthusiasts.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss in Canada dating back over 4 billion years. The distinct alternating light and dark bands are created by mineral segregation during metamorphic recrystallization.
Field identification & locations
Identified by its visible mineral grains (quartz, feldspar, biotite) and subtle parallel banding or streaking, along with its smooth, stream-worn rounded pebble shape. Commonly found along riverbeds, beaches, and glacial till deposits worldwide.
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