metamorphic
Banded gneiss pebble
Gneiss, a foliated metamorphic rock composed mainly of quartz, feldspar, and dark biotite or amphibole
AI-generated identification · may be incorrectGray and cream mineral bands with a coarse, crystalline texture; minerals are typically Mohs 6–7, with vitreous quartz and feldspar and darker minerals of variable luster. The rounded shape is from abrasion, not cleavage.
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Physical properties
Gray and cream mineral bands with a coarse, crystalline texture; minerals are typically Mohs 6–7, with vitreous quartz and feldspar and darker minerals of variable luster. The rounded shape is from abrasion, not cleavage.
Formation & geological history
Forms when pre-existing rocks undergo high heat and pressure, producing coarse mineral segregation into bands; many gneisses are Precambrian, though this pebble’s age cannot be determined from the image.
Uses & applications
Used as crushed stone and dimension stone; attractive banded pieces are also collected or used decoratively.
Geological facts
The visible wavy light-and-dark pattern is characteristic gneissic banding, formed by mineral recrystallization and segregation.
Field identification & locations
Look for alternating light quartz–feldspar and dark mica or amphibole bands; rounded pebbles commonly occur in river or beach deposits derived from gneiss-bearing terrain. Exact identification requires close inspection.
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