Abstract: The refractive index of gem-quality tourmaline is usually 1.635~1.663 (N _o) and 1.610~1.630 (N _e), and the tourmaline whose refractive index is larger than 1.663 (N _o) and 1.630 (N _e) is rarely seen in market. In this paper, a tourmaline, whose refractive index is 1.640~1.672, has been studied to prove that the high content of iron is the cause of its high refractive index.