Identification of Chinese fresh-water pearls using Mn2+ activated cathodoluminescence

Fresh-water tissue graft-cultured-pearls from China were investigated using hot cathode and cold cathode cathodoluminescence microscopes. Supplementary investigations were done using X-radiography, a Scanning Electron Microscope, an optical microscope, and an Atomic Force Microscope (AFM). The results were compared with those of natural fresh-water pearls from the Mississippi river. Thin sections of fresh-water natural pearls from the Mississippi river could be distinguished from those of fresh-water tissue graft cultured pearls from China using a hot cathode cathodoluminescence microscope according to the distribution of Mn2+ as follows: (1) Fresh-water tissue graft cultured pearls from China contain domains of calcite, emitting orange cathodoluminescence in the central region, which are almost absent in the natural fresh-water pearls from the Mississippi river. (2) The concentration of Mn2+ in natural fresh-water pearls from the Mississippi river is marked by regular green (lambda = 566 nm) zones of CL from the middle up to the periphery, whereas in case of the tissue graft fresh-water pearls from China the Mn2+ concentration decreases from the middle towards the periphery of the pearl as it is revealed by the gradually diminishing of CL towards the periphery. Whole specimen of the two types of pearls mentioned above can be distinguished from one another without destroying the samples using a cold cathode cathodoluminescence microscope as follows: The intensity of CL at 566 nm emitted from the surface of natural fresh-water pearls from the Mississippi river is higher than the intensity of CL emitted from the surface of fresh-water tissue graft cultured pearls from China, due to the fact that the surface layers of the former pearls contain more Mn2+ than those of the later. A further criterion of the Chinese tissue graft cultured pearl is the utmost smoothness of their surfaces. Moreover according to AFM observation the size of the aragonite crystals on the surface of the Chinese fresh-water tissue graft cultured pearls is much smaller than those of natural fresh-water pearls from the Mississippi river.