The hue of a tourmaline from the Umba Valley, Tanzania, changes from green to red with increases in path length of light through the material. This hue change phenomenon, called the 'Usambara effect', is different from any colur-change effect previously described in gemstones. Colorimetric methods have been used to analyze this tourmaline. Results of this study suggests that the Usambara effect is a complex phenomenon including effects of both path length and the colour change observed under different light sources (the alexandrite effect).