Folded strata in gneiss. Polished slab of folded gneiss rock. Gneiss is a metamorphic rock, one formed at high temperatures and pressures deep within the Earth's crust. The high pressures produce the folds in the banding seen here. The lighter coloured layers are formed by feldspar and quartz, while muscovite, biotite and hornblende are commonly abundant in the darker bands. This sample is 13 centimetres across.