Granite Gneiss

Physical Properties

Hardness: 6-7 Mohs; Color: Salt-and-pepper or pinkish/gray with visible layering; Luster: Dull to vitreous; Crystal structure: Foliated with alternating bands of light and dark minerals; Cleavage: Poorly defined outside of mica minerals.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous granitic precursors (orthogneiss) under high temperature and pressure, typically during mountain-building events (orogenies).

Uses & Applications

Used widely in construction for building stone, floor tiles, paving, and occasionally as ornamental stones or landscaping material due to its durability.

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 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten.'

Field Identification & Locations

Identify in the field by looking for 'gneissic banding'—alternating light (quartz/feldspar) and dark (biotite/amphibole) layers. It does not split as easily along planes as schist does.

Identified on: 4/30/2026

Mode: Standard