Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray, white, and black with visible 'salt and pepper' banding; Luster: Dull to pearly; Crystal structure: Course grained with distinct foliation (gneissose banding); Cleavage: Poor, usually breaks along uneven surfaces; Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is subjected to intense heat and pressure, causing minerals to recrystallize and segregate into bands. Often Precambrian to Paleozoic in age.

Uses & Applications

Used primarily as a dimension stone for flooring, ornamental building facades, gravestones, and as crushed stone for road construction and landscaping.

Geological Facts

Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada which is roughly 4.03 billion years old. The term 'Gneiss' comes from an Old High German verb meaning 'to spark'.

Field Identification & Locations

Identify in the field by looking for 'gneissose banding'—distinctive light and dark layers of minerals that are thicker and more irregular than the layers in schist. Commonly found in the cores of mountain ranges and ancient continental shields.

Identified on: 4/25/2026

Mode: Standard