一种仿海蓝宝石（染色石英岩）的特征分析

近期,北京北大宝石鉴定中心在日常检验中陆续收到了一些自称是“海蓝宝石”的样品,尤以素面,刻面手串常见.采用常规仪器对该样品的折射率,密度,硬度等参数进行检测.结果表明,该送检样品的折射率约为1.54,相对密度为2.66,摩氏硬度为7.虽然样品的颜色,外观,密度,硬度都与海蓝宝石的接近,但其放大观察特征与海蓝宝石的略有不同.为了确定其真伪,采用傅里叶变换红外光谱仪对样品进行了测试分析.结果表明,该样品是由石英组成,为染色石英岩.该仿制品虽然在外观上与海蓝宝石很相似,常规宝石学参数也相似,但是其放大观察特征与海蓝宝石有所区别.建议检验机构在对海蓝宝石的鉴定中应重点观察其放大特征及偏光镜下特征,如果发现疑点,应引起警惕,采取其它手段进一步测试.

Study on Gemmological Characteristics of an Imitated Aquamarine

Recently, Gem Appraisal Center of Peking University in daily inspection has re- ceived some samples claimed to be ＂aquamarine＂ . The shapes of samples are various, such as cabochon and faceted bracelets. Aquamarine is a light blue, green blue to blue-green ber- yl, an important gem of the beryl family. The English name of aquamarine comes from Lat- in, meaning ＂seawater＂, whieh indicates its resemblance to the colour of seawater. Aquama- rine wins people＇s affection with its elegant, beautiful sky blue colour. It has been popular since the middle ages, and has a long history. The gem industry also declares it as the birth- stone of March, symbolizing composure, braveness and courage. Due to the price increase of natural aquamarine recently, there is a large variety of imitations in the market, most of which can be easily distinguished from aquamarine. But the samples in this paper have a rela- tive high degree of similarity. First, conventional instruments are adopted to test refractive index, relative density and hardness of the samples. Results show that the samples have re- fractive index of 1.54, relative density of 2.66 and Mohs＇ hardness of 7. Although these conventional gemmological parameters of the samples are similar to those of aquamarine, the characteristics in magnification observation are different for both. The magnification obser-vation shows that blue colour in the samples is distributed along the fractures, which is non- directional. The distribution characteristics of colour in the samples are caused by dyeing. It is important to note that the natural fractures in the aquamarine is also common, which is hard to be distinguished from that of dyeing samples by naked eye. Therefore, more careful observation is required. The fractures in natural aquamarine are usually oriented, and colour is well distributed. The samples under polariscope show all-bright phenomenon, an indica- tion of a heterogeneous aggregation. It is clearly different from aquamarine, a member of hexagonal system, which shows ＂four bright and four dark＂ phenomenon. In order to deter- mine its authenticity, Fourier transform infrared spectrometer is used to analyze the sam- ples. Reflection method is used for their mineralogical species and the results show that the mineral component is quartz. Combining the previous test results, it is concluded that the samples are dyeing quartzite. Compared with the general dyeing quartzite, the dyeing quality of the samples are obviously much better. Since quartz class exists widely in the crust, it is cheap, easy to obtain, and is often used to imitate all kinds of gemstones. Gemmologists should always pay attention to them. Although the samples in this paper are similar to aqua- marine in appearance, as well as conventional gemmological parameters, some of its magnifi cation characteristics are different from those of aquamarine. The authors suggest that in- spection institutions should focus on the identification characteristics of aquamarine, by mi- croscope and polariscope. If suspicious feature is noticeable, other testing methods should be used.