Gneiss

Hardness: 6-7 (Mohs scale), Color: Banded grey, white, and brownish, Luster: Dull to sub-vitreous, Structure: Coarse-grained with distinct foliation/banding, Cleavage: Poor, Specific Gravity: 2.6-2.9

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. Deeply buried in the Earth's crust, often during mountain-building events (orogenies).

Used as a dimension stone for flooring, cemetery markers, facing stone on buildings, and as crushed stone for road construction and railroad ballast.

Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term 'gneiss' comes from an Old High German word meaning 'to spark' or 'bright'.

Identified by 'gneissose banding' consisting of alternating layers of light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, amphibole). Found in the cores of mountain ranges and eroded shield areas. Tip: Bands are usually thick and wavy compared to schist.