metamorphic

Banded gneiss

Gneiss, typically quartz–feldspar–mica/amphibole assemblage

AI-generated identification · may be incorrect

Coarse-grained, strongly light–dark banded, with elongated mineral streaks and a generally dull to slightly vitreous luster; hardness about 6–7 Mohs. Bands reflect alternating quartz/feldspar-rich and biotite- or amphibole-rich layers; cleavage is poor overall, though mica layers may split.

Identified September 3, 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 to Paleozoic mountain-building events. Intense heat and pressure segregated minerals into compositional bands and stretched them along foliation.

Uses & applications

Used as dimension stone, architectural facing, paving, landscaping rock, and aggregate; attractive polished pieces are collected but usually have modest commercial value.

Geological facts

The specimen’s foliation and gneissic banding distinguish it from massive granite; dark bands may contain biotite or hornblende, while pale bands are mainly quartz and feldspar.

Field identification & locations

Common in ancient continental shields and exposed mountain belts worldwide, including Scandinavia, Canada, the United States, India, Brazil, and Africa. Look for alternating coarse mineral bands, testable quartz hardness, and mica sparkle along foliation.