Gneiss

Gneiss is a metamorphic rock — it is not igneous and not sedimentary. It forms when a pre-existing rock is transformed in the solid state by high temperature and pressure, deep within the Earth's crust, until its minerals recrystallize and reorganize into bands. The original rock is not melted; instead, its grains grow larger and migrate into light and dark layers while remaining solid. That makes gneiss the product of metamorphism, the third great rock-forming process alongside igneous (crystallized from melt) and sedimentary (deposited and cemented) processes.

Gneiss can be born from more than one kind of parent. When a granite or other igneous rock is metamorphosed, the result is called orthogneiss; when a sedimentary rock such as shale or sandstone is the starting material, the result is a paragneiss. Either way, the rock you end up with is metamorphic. The most common confusion is with granite, an igneous rock that contains almost identical minerals — but granite's crystals are arranged randomly with no banding, whereas gneiss has clearly segregated light and dark layers.

Gneiss forms under high-grade metamorphic conditions — broadly higher temperatures and pressures than those that produce slate, phyllite or schist. As a parent rock is buried deep beneath mountain belts and heated, its minerals begin to recrystallize and the elements within them migrate over short distances. Light-colored, felsic minerals (quartz and feldspar) and dark-colored, mafic minerals (biotite, amphibole) gradually separate into distinct bands. This sorting, called metamorphic differentiation, produces the characteristic gneissic banding.

Because these conditions occur where continents collide and crust is thickened, gneiss is typical of the deep roots of mountain ranges and of ancient continental shields. With continued metamorphism a schist can be converted into a gneiss as its grains coarsen and its minerals segregate; pushed even further, toward the point where the rock begins to partially melt, gneiss grades into migmatite, a "mixed" rock of swirled metamorphic and once-molten material. Gneiss thus sits near the high-temperature end of the foliated metamorphic sequence, just short of melting.

The defining feature is banding. Look for alternating light and dark stripes — pale layers of quartz and feldspar separated by darker layers of biotite or amphibole — running in roughly parallel bands across the rock. The grains should be coarse enough to see with the naked eye, giving the rock a grainy rather than smooth feel, and the overall stone is hard: the quartz and feldspar will scratch glass. The bands may be straight, wavy or folded, but they are always present and visible.

The most important test is how the rock breaks. Unlike schist, gneiss does not split readily along its bands. Despite the layering, a hammer blow tends to fracture it across the banding into blocky chunks rather than peeling it into sheets, and the dark bands lack the strong satiny or sparkly sheen of a mica-rich schist. Use the contrasts to confirm: granite has the same minerals but no banding at all; schist is finer, far more micaceous, splits easily and glitters; and a rock with chaotic, swirled, partly fused-looking layers is more likely migmatite. Coarse grain plus segregated bands plus a tough, non-splitting body equals gneiss.

Gneiss is hard, dense and resistant to weathering, so its main uses overlap with those of granite. Cut and polished, its sweeping bands make an attractive, durable dimension stone for countertops, floor and wall tiles, facing slabs and cladding; in the stone trade some banded gneisses are even sold under the broad label of "granite." Blocks of gneiss are used for facing and building stone where a striking layered pattern is desirable.

Crushed gneiss also serves as a sturdy construction aggregate for concrete, road base and fill, much like other hard crystalline rocks. One practical caution distinguishes it from granite: because its minerals are arranged in layers, gneiss can be slightly weaker along the banding and may split or wear preferentially in that direction, so it is selected and oriented with that grain in mind when used as load-bearing or heavily trafficked stone.