gems, gemology, phenomena, color change, usambara effect

Crystals of green chrome tourmaline from the Umba Valley (Tanzania) may show a wine-red colour when viewed in certain directions. This is neither pleochroism, nor a colour-change of alexandrite type. The change from green to red depends primarily on the path length of light through the stone and probably also on the content of Cr and V; in the samples tested the stones appear red when more than 15 mm thick. It is suggested that this new type of colour change be called the ‘Usambara effect’.