Gneiss

Hardness: 6-7 (Mohs scale), Color: Banded grey, pink, brown, and black, Luster: Dull to vitreous, Crystal structure: Coarse-grained foliated, Cleavage: Poorly developed, Specific gravity: 2.6-2.9

Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure deep within the Earth's crust. It is often found in the cores of mountain ranges.

Used primarily as a dimension stone for flooring, cemetery markers, facades, and as crushed stone for road construction and landscaping.

Gneiss is one of the oldest rock types on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years ago. The distinctive banding is called 'gneissic banding'.

Identified by its alternating layers of light-colored (felsic) and dark-colored (mafic) minerals. Found in continental shield areas and mountain belts. Collectors look for well-defined folding patterns in the bands.