The term ＇＇alexandrite effect＇＇ is used in gemology to describe a distinct change of color appearance when a gemstone is switched from daylight to incandescent light. Gemstones that exhibit the alexandrite effect can actually be divided into four categories based upon the value of the calculated absolute hue-angle change of the material under different pairs of illuminants. The alexandrite effect is a non-color-constancy phenomenon. It can be explained by a combination of both chromaticity adaptations to the different light sources, and vision system responses to the spectral distribution of the light emitted by alexandrite effect gemstones when they are illuminated alternatively by the different light sources.