Abstract: A variety of Chatham flux-grown ruby with the dark coloured cores developed by the Chatham Company is available in gem markets recently. The internal feature of this stone is characterized by the absence of the flux residues and by the general growth of the dark coloured cores. The study shows that Chatham flux-grown ruby has well-deve loped symmetry growth zoning structure, which is the thin hexagonal layers dis tributing alternatively and the black core with zoning structure. The absorption edge of the 0^2-—Fe³⁺ charge transfer band within the ultraviolet zone is smaller than 230 nm. In addition, the absorption tail of this charge transfer band is smaller than 320 nm. It has been concluded that the change of the physicochemical condition and the heterogeneous local concentration of impurity elements of Cr and Ti during the growth process of the ruby are the main causes for the formation of the dark coloured cores.