Gemmological and Spectroscopic Characteristics of Blue Sapphire from Madagascar

Madagascar sapphires currently occupy a significant portion of the jewelry market, however, there is still a need for additional information on their gemmological and spectroscopic characteristics. The gemmological and spectroscopic characteristics of sapphires from Madagascar in this study were investigated by using conventional gemmological test instruments, photomicrograph, laser ablation inductively coupled plasma spectrometer, laser Raman spectrometer, Fourier transform infrared spectrometer, and microscopic UV-Vis-NIR spectrometer. Sapphires from Madagascar in this study are blue with different shades. The crystal morphologies are hexagonal short prism, barrel-shaped form and plate-shaped form, and there are triangular growth mounds and pits on the surface of the rough stone. Magnified observation reveals that the parting is well developed, and there are a large number of healed cleavages, oriented needle-like hematite accompanied with ilmenite inclusions, zircon and hematite solid inclusions, and gas-liquid inclusions inside the sapphire. The results of LA-ICP-MS show that the trace elements are characterized by high Fe and Ga and low Mg and Cr, and the Fe/Ti, Cr/Ga, Fe/Mg, Ga/Mg, and Ga/Al ratios are all biased toward the geochemical characteristics of basaltic sapphires. Infrared spectra show weak 3 309 cm^-1 peak, and strong 2 125 cm^-1 and 1 990 cm^-1 peaks which related to diaspore. These indicate that the sapphire has not been heat-treated. UV-Vis-NIR absorption spectra indicate that the color of sapphires from Madagascar are related to individual Fe³⁺, Fe³⁺-Fe³⁺ pair d electron leap, and Fe²⁺-Ti⁴⁺ intervalence charge transfer. The absorption at 330 nm commonly called "shoulder", which is closely related to the higher Fe³⁺ element, but the low elemental Ti content causes less vivid color.