1978 年 5 巻 4 号 p. 147-156
Recently black pearls have been successfully cultivated in the Ishigaki Islands, Okinawa, and also in Tahiti Islands. Blue pearls have also been obtained occassionally among ordinary cultivated pearls. These have already appeared in the market. On the other hand, artificial colouring of pearls to obtain black and blue pearls has been performed and the products have been in the market since 1930. It has therefore become an urgent need to establish non-destructive methods to differentiate between true and artificially coloured black and blue pearls. To achieve this purpose, both true and artificially coloured black and blue pearls are investigated by means of (1) observation of fluorescence under ultraviolet light, (2) observaton under the polarization microscope, (3) investigation of different colouring on infrared film, (4) X-ray fluorescence analysis (black pearls), and (5) fiberscopic observation (blue pearls). It was found that true and artificially coloured black pearls may be easily distinguished by either a combination of four different methods or simply by comparing different colouringi on infrared films, but in the case of blue pearls, further studies are necessary. The definition of black and blue colors, origin of the colours, the methods of cultivation of black pearls are also described in some details.