Labradorescence is a complex optical phenomenon that can be found in certain minerals, such as Labradorite or Spectrolite. Be- cause of their unique colour properties, these minerals are often used as gemstones and decorative objects. Since the phenomenon is strongly orientation dependent, such minerals need a special cut to make the most of their unique type of colourful sheen, which makes it desirable to be able to predict the final appearance of a given stone prior to the cutting process. Also, the peculiar prop- erties of the effect make a believable reproduction with an ad-hoc shader difficult even for normal, non-predictive rendering purposes. We provide a reflectance model for labradorescence that is di- rectly derived from the physical characteristics of such materials. Due to its inherent accuracy, it can be used for predictive rendering purposes, but also for generic rendering applications.