Gneiss is a high-grade, foliated metamorphic rock. It differs from lower grade foliated metamorphics, such as slate, phyllite, and schist, by having alternating bands/stripes of light & dark minerals. Metamorphism has been so intense that minerals have self-segregated into discrete zones. Most gneisses form by metamorphism of schists or granites.
Gneiss (6.8 cm across) - this is a quartz-biotite gneiss; the whitish-gray bands are quartz (SiO2) & the black bands are biotite mica.