Gneiss (specifically Banded Gneiss)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray, tan, and dark black bands; Luster: Vitreous to dull; Structure: Foliated (gneissose banding); Cleavage: Poor, usually fractures along planes; Specific Gravity: 2.6-2.9

Formation & Geological History

Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks at high temperatures and pressures. Often associated with Precambrian orogenic (mountain-building) events.

Uses & Applications

Used in construction as crushed stone for road bases and concrete aggregate. High-quality slabs are used for countertops, floor tiles, and building veneers.

Geological Facts

Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from an Old High German word meaning 'to spark' or 'bright'.

Field Identification & Locations

Identify in the field by its distinct alternating layers of light-colored (felsic) and dark-colored (mafic) minerals. Common in the cores of mountain ranges and crystalline shields.

Identified on: 4/24/2026

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