Gneiss

Gneiss is what schist becomes when it gets hotter. The mica that gave schist its sheen is largely consumed, the grains coarsen, and the rock separates into alternating pale and dark bands — feldspar and quartz in one, the darker minerals in the other. That banding is the identifying feature, and unlike schist a gneiss does not split along it.

It is the characteristic rock of the shield areas: the ancient, deeply eroded cores of continents where the roots of long-vanished mountain ranges are exposed at the surface. Some of the oldest rock known on Earth, in northern Canada, is gneiss approaching four billion years old.

Gneiss forms at the upper end of metamorphism, close to the point where rock begins to melt. Minerals of different composition become unstable together and separate into layers, a process called metamorphic differentiation, and the result is banding that looks deceptively like sedimentary bedding while having nothing to do with it.

It can be reached from either direction. A sedimentary parent — mud, then slate, then schist, then gneiss — gives a paragneiss; an igneous parent, most often granite, gives an orthogneiss. Telling them apart requires chemistry rather than appearance, and the distinction matters because it decides what the rock was doing before the mountain belt found it.

Push the temperature a little further and the pale bands start to melt while the dark ones do not. The result is migmatite, a rock caught midway between metamorphic and igneous, and the point at which the categories stop being clean.

Most gneiss sold commercially is sold as granite. The dimension stone trade uses the word for any hard, coarse, polishable silicate rock, and a banded gneiss with a strong pattern often commands more than a genuine granite because the banding is what a buyer is looking at.

Its other significance is as ground rather than as product. Shield gneiss is old, cold, dry and structurally stable, which is why it is repeatedly proposed as a host for deep geological disposal of nuclear waste — the rock's argument is precisely that nothing has happened to it for a very long time.