metamorphic

Banded gneiss (likely granitic gneiss)

Foliated gneiss composed mainly of quartz, feldspar, and dark biotite/amphibole

AI-generated identification · may be incorrect

Hardness about 6–7; gray to greenish-gray with alternating light and dark bands, granular texture, and vitreous to dull luster. Foliation is visible as planar mineral alignment; cleavage is poor, while quartz and feldspar show irregular fracture.

Identified September 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or another felsic rock underwent high-grade regional metamorphism, typically under intense heat and pressure during mountain building. Its minerals commonly recrystallized hundreds of millions to billions of years ago, depending on locality.

Uses & applications

Used as crushed aggregate, dimension stone, landscaping rock, and occasionally decorative facing stone; this small unpolished specimen has minimal commercial value.

Geological facts

The light bands are quartz- and feldspar-rich, while dark bands contain biotite or amphibole. Gneissic banding can preserve evidence of ancient continental collision and deep crustal deformation.

Field identification & locations

Identify it by alternating coarse light/dark bands and a hard, crystalline texture; it commonly occurs in exposed Precambrian shields and metamorphic mountain belts worldwide. A magnet or streak test may help detect abundant iron-bearing dark minerals.