Banded gneiss cobble

Typically gray to tan with alternating light and dark compositional bands, granular texture, and vitreous to dull luster. Hardness is about 6–7 Mohs; crystals are interlocked, with weak foliation and no dominant cleavage.

Formed when granite or other felsic rock underwent high-grade regional metamorphism, commonly during Precambrian mountain-building; the specimen’s exact age cannot be determined from the image. Compositional banding developed through mineral segregation and deformation under intense heat and pressure.

Used as dimension stone, paving, aggregate, landscaping rock, and occasionally decorative slabs. Attractive strongly banded pieces are collected, though ordinary cobbles have modest value.

Gneiss is identified by mineral banding rather than the thin, platy foliation typical of schist. Quartz and feldspar form pale bands, while biotite or amphibole commonly produces darker layers.

Look for alternating coarse-grained light and dark bands that continue across fractured surfaces, plus a very hard, massive texture. Common in Precambrian shield regions and metamorphic mountain belts worldwide; likely value is $5–$30 for a typical hand specimen, or $50–$200 for an attractive polished piece.