Gneiss

Hardness: 6-7 Mohs; Color: Black, grey, and white; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Subjection to intense heat and pressure causes minerals to segregate into bands. Often Precambrian in age.

Used in construction as crushed stone for road bases, as dimension stone for flooring and gravestones, and as decorative landscaping rock.

Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is a result of chemical reactions at high temperatures and pressures.

Identified by the alternating layers of light-colored minerals (feldspar, quartz) and dark-colored minerals (biotite, hornblende). Common in the roots of mountain ranges and eroded crystalline shields. Look for 'zebra stripes' that do not split easily like schist.