Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark grey, cream, pink, and tan; Luster: Dull to pearly; Crystal structure: Coarse-grained with distinct foliation (layering); Cleavage: Poorly defined; Specific gravity: 2.6-2.9

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. This occurs deep within the Earth's crust, often during mountain-building events (orogenies).

Uses & Applications

Used primarily as a dimension stone for flooring, wall facings, and gravestones. It is also crushed for use as road aggregate and railroad ballast in construction.

Geological Facts

Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The banding is caused by the segregation of light (felsic) and dark (mafic) minerals.

Field Identification & Locations

Identify in the field by looking for 'gneissic banding' where minerals are separated into dark and light layers. It is ubiquitous in the cores of mountain ranges and stable continental shields like the Canadian Shield.

Identified on: 4/29/2026

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