Opalescence is a phenomenon seen in dispersed systems with less opacity. It occurs when white surface has iridescence. The material may be white and less opaque but appears yellowish-red in transmitted light and appears blue in scattered light. Stone structures with spherical formations of silicon create spaces in between and refract white light separating the spectral colours of different wavelength. Opalescence is the cause of play of colour in opal gemstones.  It is also a form of Tyndall effect.