Opalescence

Similar to  iridescence,  opalescence  is  an  effect  seen  in  some  gemstones,  primarily  Opals (hence its name) which causes the gem  to exhibit a shimmer of different colours when  viewed from different angles. Opalescence is  caused  by  the  reflection  of  short  wavelength  light.  It  usually  occurs  in  the  blue  spectrum,  and due to this reflection of light a milky blue,  pearly appearance occurs.

Opalescence should not be confused with play  of  colour.  The  latter  is  due  to  the  structural  pattern of Opals: because the light doesn’t have  to bend through gaps within the gem’s structure,  no diffraction takes place.