Spectroscopic Characteristics of a Natural Diamond Suspected of Synthetic Diamond
ZHU Hong-wei1, CHENG You-fa1, CHEN Shu-xiang2*, FAN Chun-li1, LI Ting1, LIU Hai-bin1, ZHAO Xiao-xue1SHAN Guang-qi1, LI Jian-jun1
1. Shandong Institute of Metrology，National Gold & Diamond Testing Center/Shandong Key Laboratory of Metrology and Measurement, Jinan 250014, China
2. Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
Abstract：Our research team recently found a natural diamond suspected to be a synthetic diamond during routine testing. The weight of the stone was 0.029 5 g (0.14 ct), the size of which was 3.32 mm×3.33 mm×2.08 mm, the color grade was H, the clarity grade was SI1, and there was abnormal extinction under the orthogonal polarizer. The sample had strong green-yellow fluorescence under short wave ultraviolet lamp, presented strong green-yellow phosphorescence, and the phosphorescence duration was more than 50 s. Fourier transform infrared spectrometer confirmed that the sample wasⅡa type, there was no obvious absorption peak between 1 400~400 cm-1, and there was an absorption peak caused by C—C lattice vibration between 1 970~2 500 cm-1. UV-VIS Spectrometer did not detect the 415 nm absorption peak, but 270 nm absorption peak could be observed. The above characteristics were suspected tobe an HPHT synthetic diamond. In order to confirm the sample formation, some spectral tests were carried out. The luminous image of the sample was tested by using Diamond-viewTM from De Beers. The testing showed that the luminous image of the sample was lizard skin and honeycomb. The constant temperature Photoluminescence spectrum of the sample found absorption peaks at 415, 428 and 450 nm under 405 nm excitation source. The peaks of 415, 428, 450 and 575 nm caused by nitrogen-vacancy Center (N-V)0 could be obtained, but 637nm absorption peak caused by nitrogen vacancy Center (N-V) was not found under the 365 nm excitation source. Ata low-temperature liquid nitrogen condition, the photoluminescence absorption peaks of 575 and 637 nm caused by nitrogen-vacancy centers (N-V)0 and (N-V)- could be determined by using 488 and 514 nm excitation sources, and the 575 nm peak intensity was much stronger than 637 nm peak intensity. The latest research results were summarized and compared: The 415 nm characteristic peak is an important characteristic of a natural colorless diamond. The diamond synthesized by CVD has a characteristic photoluminescence peak of 737 nm. HPHT synthetic diamond has characteristic photoluminescence peaks of 882 and 883 nm. The HPHT-treated diamond has 575 and 637 nm photoluminescence characteristic peaks, and the intensity of the 637 nm photoluminescence peak is much greater than that of 575 nm. By comparing the luminous image characteristics of natural, synthetic and treated diamonds tested by Diamond-viewTM, the luminous image of the test sample is consistent with the characteristics of natural diamonds. Based on the above results, it was finally confirmed that the sample was a natural Ⅱa-type diamond. The study of this sample showed that the identification of the diamonds needs to start from the standard gemological characteristics, and pay attention to the test of the infrared absorption spectrum and ultraviolet-visible spectrum, especially to the Diamond-viewTM fluorescence imaging and photoluminescence spectrum analysis. Only a comprehensive judgment through the above methods can obtain an accurate result.
Key words：Natural diamond; Type Ⅱa; Fluorescence; Photoluminescence; Phosphorescence
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