Gneiss

Typically medium- to coarse-grained, gray, and strongly banded or streaked from alternating quartz/feldspar-rich and dark mica/amphibole-rich layers; Mohs hardness about 6–7, vitreous to dull luster, with weak cleavage controlled by foliation. The specimen appears weathered and may also be a coarse banded granite or granitic gneiss.

Formed when granite or other felsic rock undergoes intense regional metamorphism at roughly Г500–800°C and elevated pressure, causing mineral segregation and foliation; individual specimens may be Precambrian to much younger.

Used as dimension stone, building aggregate, paving and landscaping stone, and occasionally as decorative slabs; attractive banded pieces are collected but are rarely gemstones.

Quartz and feldspar produce the pale matrix, while biotite or amphibole create dark bands. Gneiss commonly forms deep in mountain belts and is exposed after uplift and erosion.

Look for compositional banding, interlocking grains, and a hard, granular texture; unlike slate or schist, it generally has thicker, less easily split bands. Common in ancient continental shields and metamorphic mountain belts worldwide.