metamorphic

Banded gneiss

Quartzo-feldspathic biotite gneiss, possibly garnet-bearing

AI-generated identification · may be incorrect

Typically medium- to coarse-grained with alternating light quartz–feldspar and dark biotite/hornblende bands; hardness about 6–7, vitreous to pearly luster, nonmetallic, and strong foliation with irregular fracture. The rusty orange areas are likely iron-oxide staining.

Identified September 12, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed by high-grade regional metamorphism of granite or sedimentary protoliths, commonly during Precambrian mountain-building; exact age cannot be determined from an image. Intense heat and pressure segregated minerals into compositional bands.

Uses & applications

Used as dimension stone, paving, aggregate, and decorative stone; attractive pieces may be collected, though this rough specimen has limited commercial value.

Geological facts

Gneiss banding reflects metamorphic mineral segregation rather than ordinary sedimentary layering. It commonly contains quartz, plagioclase or potassium feldspar, mica, and amphibole, with accessory garnet or magnetite.

Field identification & locations

Look for planar light-dark banding, visible interlocking grains, and mica flakes that may glitter or peel along foliation; it is commonly found in ancient continental shields and deeply eroded mountain belts. Typical value is about $5–$30 for a hand specimen, or roughly $20–$100 if unusually attractive or locality-documented.