metamorphic

Banded gneiss pebble

Biotite–quartz–feldspar gneiss (composition variable)

AI-generated identification · may be incorrect

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.

Identified August 15, 2026More metamorphic →

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Physical properties

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.