Gneiss

Hardness: 6-7 on Mohs scale; Color: Gray, black, and tan banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-2.9

Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks, where intense heat and pressure cause minerals to segregate into bands. These are often found in Precambrian shields.

Commonly used as a dimension stone for flooring, gravestones, and facing for buildings; also used as crushed stone for road construction and landscaping.

Gneiss consists of the same minerals as granite but shows distinct structural banding. Some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, are over 4 billion years old.

Identified by 'gneissic banding'—alternating light and dark layers of minerals like quartz, feldspar, and mica. It is common in mountainous regions and glacial till. Field identification relies on observing the coarse, layered texture that doesn't split easily like schist.