Detection of Natural and Synthetic Emerald by Using Infrared Spectroscopy–ATR,Specular Reflection,Transflectance and Diffuse Reflectance Techniques

Emerald is a green(Cr-bearing)variety of beryl.It has also been one of the highest value and highly demanded gemstones in the world.For this reason,the methods for synthesis emerald(i.e.,hydrothermal,flux grown)have been constantly developed.In many cases,it is hard to distinguish the natural emerald from the synthetic one using the basic instruments.Therefore,with the advantage of non-destructive technique,the infrared spectroscopy has been efficiently applied for the task.In this study,to find the potential technique in FTIR spectroscopy for classifying between natural and synthetic emerald(hydrothermal and flux-grown),the observed spectra from the Attenuated Total Reflectance Radiation(ATR),Diffuse Reflectance(DRIFT),transflectance,and specular reflection techniques were compared.The result showed that the spectra obtained from the transflectance and the DRIFT techniques were similar with equal spectral qualities.Unlike the diffuse reflectance spectra,the transflectance spectra were not affected by the sample orientations.The spectral features associated with water molecule in the crystal structure were clearly observed in both the DRIFT and transflectance spectra.Although the spectral features associated with weak absorption bands of water were not observed in both the DRIFT and transflectance spectra,the position of strong absorption band at 3 800-3 500 cm^(-1) and 1 635 cm^(-1) could be detected in the ATR spectra.In addition,the lack of absorption of water between 4 000 and 3 400 cm^(-1) in flux-grown emerald samples is the clue to separate it from the natural and hydrothermal one.In the meantime,the absence of the strong absorption peaks between 2 300 and 1 400 cm^(-1) in hydrothermal emerald can differentiate it from the natural one.

Detection of Heat Treatment in Ruby and Sapphire by Using Infrared Spectroscopy

The detection of the heat treatment is one of the infinite problems for testing gem-corundum samples in particular ruby and sapphires.Conventionally,the alteration of inclusions by heating process can provide the reliable shreds of evidence.Though,for strengthening those clues,the infrared spectroscopy was also put into use as a supporting technique.There is a number of absorptions,i.e.,molecular water(board at 3 000-3 500 cm^(-1)),a set of significant peaks at 3 309,3 232,and 3 185 cm^(-1)(3 309 cm^(-1)-series peak)of structural-OH,and possible hydroxyl minerals such as boehmite/diaspore(1 980-2 200 cm^(-1))in FTIR spectra of ruby and sapphire samples.Those absorptions have been used as an indicator to indicate whether the sample has been undergone heating.Based on our continual researches the appearances of the peaks related mainly to the rearrangements of reactive trace elements in particular Ti^(4+) in the structure of the sample.Therefore,it is reasonable that those peaks should be assigned to-Ti-OH stretching.The ruby and sapphire samples from different geographic origins were collected.The samples were prepared by cutting in perpendicular to their c-axis for eliminating the polarization effect.Besides,they were polished into a thin slide at the thickness around 2 mm for controlling the optical path difference.In order to observe the behavior of the 3 309 cm^(-1)-series peak,the heat treatment was performed under the common practice conditions at the temperatures 800,1 000,1 200,1 400,and 1 650℃.In addition,the chemical composition was also considered.The FTIR spectra of the samples were measured inparallel with the caxis of the sample.The FTIR spectra of ruby samples show that the 3 309 cm^(-1)-series peak only appeared in the heated samples ≥1 200 ℃ when compare with the same amount of the Ti content.This set of peaks is useful to indicate the heat treatment at high temperature.In the case of heating at low temperature,the shift positions of the peaks around 1 980-2 200 cm^(-1) could be a good clue.On the other hand,the FTIR spectra of the sapphire samples show the 3 309 cm^(-1) peak with the strong absorption from the beginning.Though,the side peaks at 3 232 and3 185 cm^(-1) only appear after heating at 800 ℃ onward.In addition,the intensity of those peaks was slightly decreased until nearly absent when the samples were heated at a high tem-perature.Besides,the sample geological origins also provide considerable ideas on detection of heating;more details on this are being revealed.In conclusion,the Ti 4+content plays an important role in the occurrence of the peak at3 309,3 232,and 3 185 cm^(-1) in FTIR spectra in both ruby and sapphires.Even though,the appearance of the peaks may be different regarding differences in heating temperatures;however,the Ti content could be the first factor to be considered when using the 3309 cm^(-1)-series peak for determining whether the sample have been undergone heating.In addition,sample orientation should be strictly controlled for maximum absorption and in order to reduce the variable appearance of the 3 309 cm^(-1) peak which can lead to the uncertain or wrong or interpretation.