Gneiss (Granitic Gneiss)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Banded pink, white, gray, and black. Luster: Dull to vitreous. Crystal structure: Coarse-grained with distinct foliation (layering). Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of granite or sedimentary rocks. Subjected to intense heat and pressure deep within the Earth's crust, leading to the segregation of minerals into bands.

Uses & Applications

Used primarily as a building stone, for crushed stone in road construction, and for decorative landscape architecture and countertops (often sold as 'granite').

Geological Facts

Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The name comes from the Middle High German verb 'gneist' (to spark).

Field Identification & Locations

Identify by 'gneissose banding'—distinct light and dark layers. Commonly found in the crystalline cores of mountain ranges and ancient continental shields (like the Canadian Shield or Appalachians).

Identified on: 4/26/2026

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