metamorphic
Banded gneiss pebble
Biotite–quartz–feldspar gneiss (composition variable)
AI-generated identification · may be incorrectTypically light gray, tan, and black compositional banding; quartz and feldspar give a dull to glassy luster, while biotite forms dark layers. Hardness is about 6–7 Mohs; grains are crystalline, with weak to absent cleavage as a whole and high durability.
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Physical properties
Typically light gray, tan, and black compositional banding; quartz and feldspar give a dull to glassy luster, while biotite forms dark layers. Hardness is about 6–7 Mohs; grains are crystalline, with weak to absent cleavage as a whole and high durability.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade regional metamorphism, causing minerals to recrystallize and segregate into bands. The specimen’s exact geological age cannot be determined from an image; gneisses commonly range from Precambrian to younger mountain-building ages.
Uses & applications
Used as crushed stone, aggregate, dimension stone, and decorative landscaping material; attractive banded pieces are collected but rarely gem-grade.
Geological facts
The alternating bands record intense heat, pressure, and deformation deep within Earth’s crust. Dark bands commonly contain biotite or amphibole, whereas pale bands are quartz- and feldspar-rich.
Field identification & locations
Identify it by its coherent, irregular light–dark mineral bands and visibly crystalline texture; unlike sedimentary layers, the bands are metamorphic mineral segregation. Common in ancient continental shields and mountain belts, and frequently found as stream or beach pebbles after erosion.
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