Banded Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands of black, grey, white, or tan; Luster: Dull to vitreous; Structure: Foliated with distinct 'gneissic banding'; Cleavage: Poor, typically breaks unevenly across bands.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. This process causes minerals to recrystallize and segregate into layers. Many gneisses are from the Precambrian era, billions of years old.

Uses & Applications

Used primarily as decorative crushed stone, construction aggregate, and dimension stone for countertops or flooring when the banding is aesthetically pleasing.

Geological Facts

Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years ago. The distinct banding is a result of chemical migration during intense pressure.

Field Identification & Locations

Identify by looking for 'gneissic flowering'—parallel bands of minerals where the light bands are quartz/feldspar and dark bands are biotite/amphibole. Commonly found in mountainous regions or stable continental interiors (cratons).

Identified on: 4/28/2026

Mode: Standard