Identification of Tortoise Shell and Its Two Imitations
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摘要: 近期在珠宝市场上频繁出现两种玳瑁仿制品，一种为人造树脂制品（塑料），另一种为牛角制品（常见牛角与玳瑁拼合仿玳瑁），两者的外观特征与玳瑁相似，极具欺骗性。采用常规宝石学测试手段，结合扫描电子显微镜和傅里叶变换红外光谱仪等测试方法，对玳瑁及其仿制品的常规宝石学特征，微生长结构及红外光谱进行了综合对比分析。测试结果表明，玳瑁由无数小颗粒呈镶嵌状拼合形成层状生长结构，牛角由纤维状、针状物形成交织状生长结构。玳瑁与其仿制品的偏光性特征及内部特征等性质存在一定的差异，但折射率及密度等常规宝石学性质却较为接近。玳瑁的红外光谱特征与玳瑁仿制品（塑料）的红外光谱特征区别明显，但与牛角的差异不大，仅在1 000～1 600cm－1范围内的个别峰位存在区别。其中，玳瑁特有的N—H弯曲振动导致的1 534，1 520cm－1处的强吸收谱带及C—N伸缩振动致明显强的1 239cm－1吸收谱带可作为区分玳瑁与牛角的鉴别特征。结合微生长结构，常规宝石学特征及红外光谱可对玳瑁及其仿制品进行鉴别。Abstract: 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.-
Keywords:
- tortoise shell /
- imitation /
- microstructure /
- infrared spectroscopy
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