Banded gneiss

Typically hard (about 6–7 Mohs), gray to black and white, with strong alternating mineral bands, granular texture, and vitreous to dull luster. The banding reflects compositional layering and foliation rather than sedimentary bedding.

Formed by high-grade regional metamorphism of granite or other felsic protoliths, generally during mountain building; individual specimens may be Precambrian to Phanerozoic in age. The image may also show a strongly banded metamorphic rock such as migmatitic gneiss.

Used as dimension stone, architectural cladding, countertops, paving, and aggregate; attractive slabs are collected and occasionally used ornamentally. This small rough specimen has modest commercial value.

Quartz and feldspar commonly form the light layers, while biotite, amphibole, or other dark minerals form the darker bands. Partial melting can produce irregular, swirled bands called migmatite.

Identify it by coarse interlocking grains and persistent light–dark foliation; unlike slate or shale, it is not usually fissile into thin sheets. Common in ancient continental shields and metamorphic mountain belts worldwide, including Canada, the United States, Scandinavia, India, and Australia.