Gemmological Characteristic of Grandidierite from Madagascar

Grandidierite, one of the rarest gems, has been discovered recently in Madagascar. 8 pieces of grandidierite samples from Madagascar were collected and studied by gemmological testing methods and advanced instrumentation including energy dispersive X-ray fluorescence spectrometer, Fourier transform infrared spectrometer, and Raman spectrometer. The results showed the colour of the grandidierite is from strong bluish green to greenish blue, and its polished facet is glassy lustre. The refractive index is 1.578-1.619, and birefringence is 0.041. It's pleochroism of bluish green, deep green and colourless. Through the analysis of X-ray fluorescence spectra, it can be preliminarily concluded that the colour of grandidierite may be related to its Fe element, and the higher the Fe content, the more intense the colour. It is also characterized by infrared spectra and Raman spectra, which are related to the BO₃ triangles, SiO₄ tetrahedra, distorted AlO₅, MgO₅, and FeO₅ trigonal bipyramids, and AlO₆ octahedra. In infrared spectra, the absorption peak of vibration of BO₃ triangles is in 1 300-1 500 cm^-1 region while the absorption in 800-1 100 cm^-1 region is related to the silica skeleton. The vibration of the BO₃ triangles in the Raman spectrum is reflected in the range of 800-1 100 cm^-1. Infrared spectroscopy and Raman spectroscopy can be used as methods for grandidierite identification.