metamorphic

Banded gneiss

Gneiss, commonly composed of quartz, feldspar, and mica/amphibole

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; dark gray to brown with alternating light and dark mineral bands, generally dull to slightly sparkly, and weak or absent cleavage across the rock. The visible foliation and compositional banding are diagnostic.

Identified September 7, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or other rocks undergo high-grade regional metamorphism, causing minerals to recrystallize and segregate into bands. Many gneisses are Precambrian, commonly over 1 billion years old, though younger examples occur worldwide.

Uses & applications

Used as dimension stone, landscaping rock, riprap, aggregate, and occasionally polished architectural stone; attractive specimens are collected but are usually modestly valuable.

Geological facts

Gneissic banding records intense heat and pressure deep in the crust and may preserve evidence of an older parent rock. It commonly occurs in ancient continental shields and mountain belts.

Field identification & locations

Identify it by coarse grains and distinct light-dark, often wavy bands; unlike slate or schist, its foliation is generally not easily split into thin sheets. Common localities include Scandinavia, Canada, the northeastern United States, India, and Australia.