Gneiss

Gneiss, metamorphic rock in which the minerals have been segregated into parallel layers, creating a banded or laminated structure. The metamorphosis of many igneous and sedimentary rocks has resulted in a banded structure in which the quartz and feldspar are segregated into layers that alternate with layers of dark minerals. The different varieties of gneiss are named after the type of rock from which they have been formed (as granite gneiss and diorite gneiss) or after a mineral in which the rock is unusually rich (as biotite gneiss and hornblende gneiss).