Gneiss

Hardness: 6-7 (Mohs), Color: Banded grey, white, and green-black, Luster: Vitreous to dull, Crystal structure: Foliated/Banded, Cleavage: Poorly defined, Specific gravity: 2.6-2.9

Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature. Its distinct banding, called gneissose foliation, results from mineral segregation during deformation.

Used as a dimension stone for flooring, building facades, and grave markers; also used as crushed stone in road construction and landscaping.

Gneiss is one of the oldest rock types on Earth, with the Acasta Gneiss in Canada dated to nearly 4 billion years old. The name comes from a German word meaning 'sparkling' or 'to strike'.

Identified by alternating light and dark bands (foliation). Common in ancient continental shields like the Appalachian Mountains or the Canadian Shield. Field identification focuses on the visible mineral crystals (quartz/feldspar) and distinct layers.