Opalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals.
Effect seen in a few minerals, chiefly opal (hence its name) which cause it to exhibit a glimmer of different colors when rotated or seen in different angles. Opalescent describes mineral exhibiting this effect.