Gneiss. Credit: Mineral Information Institut.
Gneiss is a course-grained banded or foliated metamorphic rock, with poor cleavage, in which the minerals are arranged in darker and lighter layers. The composition of gneiss is variable, but often approximates to that of granite. The chief components are irregular, granular bands of quartz and feldspar alternating with thin undulating bands of micas and amphiboles.