Nacreous clouds, sometimes called mother of pearl clouds

Nacreous clouds, sometimes called mother of  pearl clouds, are rare but once seen are never forgotten. They are mostly visible within two hours after sunset or before dawn when they blaze unbelievably bright with vivid and slowly shifting iridescent colors. They are filmy sheets slowly curling and uncurling, stretching and contracting in the semi- dark sky.

Nacreous clouds are a type of Polar Stratospheric Cloud (PSC).
Beautiful but destructive. Their appearance is actually not a good thing at all: they contribute to destroying our ozone layer which protects life on Earth from harmful ultraviolet light.
Nacreous clouds form in the stratosphere, twice as high as commercial planes fly, where the ozone layer resides. Their ice crystals encourage chemical reactions between human-produced chlorofluorocarbons (CFCs), resulting in the release of chlorine gas, which breaks down ozone.

Source:http://www.atoptics.co.uk/highsky/nacr1.htm
http://www.atoptics.co.uk/highsky/psc1.htm

Image via Wikipedia Commons