metamorphic

Weathered gneissic rock

Biotite–quartz–feldspar gneiss, possibly migmatitic

AI-generated identification · may be incorrect

Coarse, irregularly banded black, gray, cream, and tan material with a granular texture and moderate vitreous-to-dull luster. Typical hardness is about 6–7 Mohs; quartz and feldspar grains show no obvious cleavage as a whole, while biotite may split into thin flakes.

Identified September 12, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock underwent intense heat and pressure, producing mineral segregation and banding; commonly Precambrian to Paleozoic in age. The rounded shape and surface staining indicate prolonged weathering or stream transport.

Uses & applications

Used locally as decorative stone, landscaping rock, aggregate, and geological teaching material; attractive polished pieces can be ornamental, but this specimen is unlikely to be jewelry grade.

Geological facts

The contrasting light and dark bands reflect quartz/feldspar-rich and mica/mafic-rich layers. If partial melting occurred, it may be termed migmatite rather than ordinary gneiss.

Field identification & locations

Common in ancient continental shields and metamorphic mountain belts; identify by coarse interlocking grains and compositional banding, unlike layered sedimentary rock. A streak, fresh broken surface, and hand lens would improve identification.