Gneiss

Typically medium to coarse grained, with alternating light and dark mineral bands, vitreous quartz, feldspar, and dark biotite or amphibole; hardness about 6–7 Mohs, no consistent cleavage, and specific gravity roughly 2.6–2.8. The specimen’s streaked, banded texture is consistent with gneiss, though image-only identification is approximate.

Forms when granite, shale, or related protoliths undergoes intense regional metamorphism, commonly at high temperature and pressure during mountain building. Many gneisses are Precambrian, commonly hundreds of millions to over 2.5 billion years old.

Used as dimension stone, building and paving material, landscaping rock, aggregate, and decorative slabs; attractive pieces are collected but usually have modest value.

Its compositional banding results from mineral segregation during deformation and recrystallization, not ordinary sedimentary layering.

Look for alternating pale quartz–feldspar bands and darker mica- or amphibole-rich layers, often with a coarse crystalline texture and foliation. Common worldwide in ancient continental shields and metamorphic mountain belts.