Gemmological Characteristics And Quantitative Study On Heat Treatment Of Ruby From Yen Bai Province,Northern Vietnam

As the center of gem mining and trading,Luc Yen of Yen Bai province in northern Vietnam hosts a deposit of ruby and sapphire with important economic value in southeast Asia.Since the discovery of Vietnamese ruby in the 1980s,its exploration and development are relatively late.Although there exists high-quality Vietnamese rubies,but the market supply is not sustainable at present.In addition,there is a lack of systematic research on the gemology and gem technology of ruby from Vietnam.In this paper,the gemmological properties and chemical composition of Vietnamese ruby were discussed in detail by means of the conventional gemology test,laser inductively coupled plasma mass spectrometry,fourier transform infrared spectrometer,laser Raman spectrometer,ultraviolet-visible spectrophotometer and micro-UV-Vis-NIR spectrometer.The results showed that rubies from Yen Bai,Vietnam belong to a typical high Cr and poor Fe type,most are with blue-violet tone and have much fracture.As the most important chromogenic element in ruby,the content of Cr is less than 1000 ppm in light purplish red rubies,1000-2000 ppm in purplish red rubies and 4000-8000 ppm in deep red rubies,respectively,which have a large span.The content of∑Fe in most of the deep red rubies in group 3 was lower than the detection limit of the instrument,while the content of∑Fe in group 4 of orange-red rubies was higher than that of other three group samples by one order of magnitude due to the orange color caused by iron.The results showed that the content of∑Fe in the red ruby was lower than that in the other samples.At the same time,rubies from Vietnam of different color showed a certain division in the Fe-Cr matching point chart.The results of infrared spectrum test indicated that all samples show five intrinsic peaks of ruby in the fingerprint region,and some of the samples have obvious absorption peaks of 2931,2110 and 1980cm^-1 caused by the stretching vibration of O-H group in diaspore.Raman spectra showed absorption peaks related to the stretching and bending vibrations of[AlO₆]group.In the UV-Vis absorption spectra,the relative intensities of Y absorption band and U absorption band were determined on the position relation between the two directions of light vibration and optical axis of crystal.When the two are perpendicular to each other,the absorption peak of the blue-purple region is higher than that of the yellow-green region.At the same time,the relative distance between the two absorption centers was closely related to the mass fraction of Cr³⁺in ruby.In addition,the inclusions in the ruby from Yen Bai,Vietnam are abundant in both species and quantity.The common material inclusions are mineral inclusions such as calcite,apatite,zircon,diaspore,rutile,etc,oriented fluid inclusions,and secondary inclusions such as iron oxides or hydroxides.Meanwhile,all of crystallographic mineral inclusions have typical Raman spectrum,most of them in Vietnamese rubies are distributed separately.Structural inclusions mainly showed fractures and cracks in rubes.The color bands in Vietnamese ruby are mainly two types:one is that there is a blue color cloud with the significance of origin along the double crystal growth surface,another type is the color zoning with gradual structure.Supported by the principles of ion redox and ion diffusion precipitation,the heat treatment experiments of different temperature,different holding time and different fluxing agent were carried out on the classified samples.The results showed that the color and local transparency of the samples increased obviously at high temperature.The optimum holding time of ruby samples with light purplish red,purplish red and deep red got longer from 6 hours to 12 hours with the increase of the color saturation.While the low temperature condition had the request to the sample choice,only keeping the holding time for over 12 hours can help improve the color of the ruby samples obviously at low temperature.Contrasting the effect of heat treatment at different stages,samples with fissure and parting were not suitable for low temperature heat treatment in terms of transparency.For the samples with more internal inclusions,high temperature heat treatment can improve the transparency of gemstones by melting more inclusions.For the samples with obvious internal yellow-brown bands,the mixture of borax and strong adsorptiveγ-Al₂O₃ can be used to help adsorb the foreign ions to remove the internal tawny bands effectively and then solve the problem of the color uniformity of samples.For samples with few fissures and off-color,the effect of low temperature heating for long time was similar to that at high temperature for short heating time,however,from the view of market acceptance,low temperature heat treatment is more easily accepted due to its fuzzy identification characteristics.Finally,based on the chromaticity theory of good reproducibility and strong objectivity,the improvement effect and color-modification efficiency of the ruby under different heat treatment conditions were described quantitatively,and proved that the color measurement system,which is a combination of spectrophotometer and color calculation software,can make up for the defects of the colorimetric stone and the Gem Dialogue colorimetric card in ruby color evaluation.In this form,the color parameters of ruby were analyzed and the effect of optimizing processing is evaluated by the quantitative data,which not only can ensure the objectivity of the description of the color,but also can assist in color classification of rubies and determine the optimum condition of heat treatment.