Gneiss

Typically medium- to coarse-grained with alternating light and dark bands or streaks. This specimen appears dominated by white to pale pink feldspar and quartz, with dark gray to black biotite and possibly amphibole. The sparkling surfaces are likely mica. Hardness is approximately 6–7 on the Mohs scale; luster is generally vitreous on quartz and feldspar and pearly to submetallic on mica. It has no characteristic cleavage as a whole rock, although its mica minerals show perfect cleavage. Specific gravity is commonly about 2.6–2.8. The irregular, foliated texture and mineral banding are characteristic of gneiss.

Gneiss forms when pre-existing igneous or sedimentary rocks undergo intense regional metamorphism at high temperature and pressure, commonly at depths of roughly 15–40 km. The minerals recrystallize and segregate into light and dark bands during deformation. Gneiss can form from granite, producing orthogneiss, or from sedimentary rocks, producing paragneiss. Individual specimens may range from Precambrian to much younger ages; many gneisses exposed at Earth's surface are hundreds of millions to several billion years old.

Used as dimension stone, building stone, paving, landscaping rock, road aggregate, retaining walls, and architectural slabs. Attractive polished varieties are used for countertops, monuments, and decorative stone. Collectors value strongly banded, colorful, or unusually coarse specimens, although ordinary hand samples are relatively common.

Gneiss is pronounced 'nice.' Its characteristic banding is not ordinary sedimentary layering; it develops through metamorphic recrystallization and deformation. Some of Earth's oldest exposed rocks are gneisses, including ancient Precambrian basement rocks. Gneiss may grade into granite, migmatite, schist, or amphibolite depending on composition and metamorphic conditions.

In the field, look for alternating light and dark mineral bands, a coarse crystalline texture, visible quartz and feldspar, and mica-rich layers. Unlike schist, gneiss usually has compositional banding rather than a uniformly strong sheet-like foliation. Unlike granite, its minerals are commonly stretched, segregated, or arranged in streaks. A hand lens can reveal flaky biotite and glassy quartz; a steel file or quartz comparison can help assess hardness. Exact identification would require a fresh surface and possibly thin-section or chemical testing, since similar-looking banded granite and migmatite are possible.