Gemological Study And Characteristics Of Origin Of Emerald From Afghanistan

Afghanistan produces a wide variety of gem minerals,of which,one of the most valuable is emerald.Emerald has beautiful and attractive color which has always been the reason to be loved and sought after in the world.In order to know the gemological properties of emerald,we selected the Afghanistan emerald as the research object and used a variety of conventional instruments to test its basic gemological features.In addition,by the help of Laser Raman spectrometer,Fourier transform infrared spectrometer?FTIS?,uv-vis spectrophotometer,chromaticit,etc.this paper illuminated the color and internal structure of the Afghanistan emerald.The result shows that,in general,the emerald mineral in Afghanistan has higher crystal transparency,and the crystal shape tends to be hexagonal column,besides,its surface has conspicuous vertical lines,which indicate the shell and ribbon fracture of Afghanistan emerald.Basing on the actual measurements on Afghanistan emerald,the range of refractive index is 1.569-1.591,and its relative density is from 2.67-2.75g/cm³.The object also shows an above-average dichroism.The content of Cr in dark part of Afghan emerald is higher than light part,the content of Al in dark part is lower than that in light part,and the content of alkali metal is consistent with that of Cr.The composition of Afghan emerald is more uniform.The test of FTIS shows that the vibration intensity of H-O-H is higher than H-O,which indicates there is certain amount of alkali metal in the object.Other characteristic peaks of Afghanistan emerald tend to be higher than those of other emeralds,which caused by elements in Afghan emeralds,such as Cr³⁺,V³⁺and Fe³⁺,which replace Al³⁺through isomorphism.It is shown in the Raman spectrometer test that all sample have the characteristics of the Raman band of the emeralds,the overall peaks are coincide.The most obvious peaks are around 685 cm^-1 and 1067 cm^-1.The fluid inclusion of the sample emerald was also detected,indicating that CO₂ was the main vapor inclusions.The uv-vis spectrophotometer test proves that the color of Afghanistan emerald is mainly due to the Cr³⁺,V³⁺and Fe³⁺in the crystal.When the content of Cr³⁺and V³⁺roughly equal,the color of emerald appears to be bright green;When the content of V³⁺was significantly higher than Cr³⁺,the color appears to be vivid green or deep green;when the content of Cr³⁺was significantly higher than V³⁺,the color appears to be light green or azure green.According to the colorimetric analysis,the main wavelength range of the color of sample is 503.629 nm-525.373 nm,and its color is different in different angles.Its hue angle is around 165°and its hue angle and saturation are negatively related to a certain extent.To observe the emerald sample carefully,we can find that the longitudinal grain development of the crystal surface which shows different interfaces with obvious color bands.It is due to the constant change in the external environment around the emerald during its formation.