Identification of Tortoise Shell and Its Two Imitations

Recently，two kinds of tortoise shell imitations appear in the jewelry market frequently.One is plastic product and the other is ox horn product or composite stone being composited by ox horn and tortoise shell.The appearances of the two tortoise shell imitations are close to that of tortoise shell，which is deceptive to consumers.In order to find out a way to distinguish tortoise shell and its imitations quickly and nondestructively，some samples were chosen for investigation.Conventional gemmological methods including refractometer，scale，ultraviolet lamp，gem microscope，polarizing microscope and modern analytical techniques including SEM，FTIR were used to find out the microstructure and spectral characteristics between tortoise shell and its imitations.
The results show that some conventional gemmological testing data of tortoise shells and its imitations are alike (RI is 1.52~1.56 and relative density is about 1.29). Only characteristics observed by long wave ultraviolet light, gem microscope and polarizing microscope exist certain differences. Under long wave ultraviolet light, the yellow part of tortoise shell shows blue and white fluorescence, and plastic shows blue and white fluorescence while ox horn shows strong blue and white fluorescence. The polarizing microscope investigation reveals the granular texture of tortoise shell and the fiber structure of ox horn, meanwhile the plastic is extinction.
The SEM images show that tortoise shell is made up of random accumulation of granule about 0.5 μm in diameter, and granules finally form lamellar structure, It also shows that ox horn's mixed structure is made up of many orientational fibers. The data of FTIR can distinguish tortoise shell and plastic product well but the difference between results of tortoiseshell and ox horn is not significant, which shows only a little difference in 1 000-1 600 cm^-1. The strong absorption bands around 1 534,1 520 cm^-1 are caused by δ(N-H)and a stronger absorption band at 1 239 cm^-1 is caused by υ(C-N), which can be regarded as their FTIR features. FTIR, microstructure and conventional gemmological characteristics should be used to get a comprehensive conclusion.