Granite Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray, white, and pinkish-red (orthoclase feldspar); Luster: Dull to vitreous with shimmering mica flakes; Crystal structure: Coarse-grained with visible banding or foliation; Cleavage: Poor in hand specimen, but distinct mineral grains show it.

Formation & Geological History

Formed through high-grade regional metamorphism of a granitic protolith. Subjected to intense heat and pressure deep within the Earth's crust, causing minerals to recrystallize and align. Often Proterozoic to Archean in geological age.

Uses & Applications

Used primarily in construction for crushed stone, road aggregate, and dimension stone for countertops or monuments. It is highly valued for its durability and aesthetic banding.

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 an Old German word meaning 'sparkling' or 'to glisten' because of its mica content.

Field Identification & Locations

Identify in the field by looking for 'gneissose banding'—distinct light and dark layers of minerals. It is commonly found in the cores of mountain ranges and ancient continental shields. For collectors, look for garnet inclusions which are common in this rock.

Identified on: 4/23/2026

Mode: Standard