metamorphic
Banded gneiss pebble
Biotite–quartz–feldspar gneiss (composition variable)
AI-generated identification · may be incorrectTypically hardness 6–7, with alternating yellowish-brown felsic bands and dark biotite/amphibole-rich layers; vitreous to dull luster, granular crystalline texture, and weak-to-absent cleavage as a whole rock. The rounded shape indicates water or glacial abrasion.
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Physical properties
Typically hardness 6–7, with alternating yellowish-brown felsic bands and dark biotite/amphibole-rich layers; vitreous to dull luster, granular crystalline texture, and weak-to-absent cleavage as a whole rock. The rounded shape indicates water or glacial abrasion.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade regional metamorphism, causing minerals to segregate into compositional bands; commonly Precambrian to Paleozoic in age. The specimen was later weathered and transported as a stream or glacial cobble.
Uses & applications
Used locally as decorative stone, landscaping material, aggregate, and polished ornamental material; attractive pieces have modest collector value.
Geological facts
The banding is called gneissic foliation and records intense heat, pressure, and deformation. Dark layers commonly contain biotite or hornblende, while light bands are rich in quartz and feldspar.
Field identification & locations
Identify it by persistent alternating light and dark mineral bands, a crystalline granular surface, and greater hardness than most sedimentary rocks; common in ancient continental shields and metamorphic mountain belts. A precise identification requires a fresh surface or thin section.
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