Gemmological Characteristic and Chemical Composition of Ruby from Guinea, Africa

The rubies originating from Guinea, Africa were studied. The gemmological characteristics of the rubies were tested by conventional gemmological instruments and the composition and spectra characteristics of the ruby sample from Guinea were analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Besides, composition characteristics of the ruby sample from Guinea were discussed and compared with those from other origins in Africa. The results indicated that the rubies from Guinea with corrosion mantle often show purple or brown hue. A great quantity of inclusions and fissilities cause low transparency of ruby from Guinea. On the basis of colour, ruby samples can be divided into two series: Purplish red and orange. With the colour from pink to dark purple, the content of Cr, Fe and the ratio of Cr/Fe show an obvious difference, which are lower than those of high quality rubies. Moreover, the content of Fe in ruby from Guinea is about 0.05%—0.15%, which can be distinguished from ruby in Kenya (< 0.001 5%), Madagascar (0.15%—0.28%) and Mozambique ( > 0.28%) but overlaps with ruby from Tanzania (0.030%—0.156%). Considering the UV-Vis absorption spectra, the purple-red series have obvious red shift phenomenon with the colour from shallow to deep, while the orange series exhibit high absorption intensity in the purple region and low absorption intensity in the green region. For the appearance of 3 310, 3 076 cm^-1 in FTIR spectra, it must be noted that only the dark red-purple samples that contain relatively high Ti would show the peak. Therefore, the 3 310 cm^-1 peak is assigned for Ti-OH. In addition, apatite, diaspora and rutile as the main crystal inclusions have been determined by Raman spectra.