Color measurement of yellow sapphire by UV-Vis reflectance spectroscopy

Abstract

Yellow sapphire is a gemstone variety of corundum (Al2O3). The yellow color is commonly caused by Fe3+ impurity substituting for Al3+ in the Al2O3 structure. The depth of yellow color is linked to Fe3+ concentration. The higher content of Fe3+ produces a deeper yellow color. However, there is low content of the impurity such as Be2+ in cooperating with the low content of Fe3+ to produce the yellow color in yellow sapphires. It is difficult to differentiate between the natural yellow sapphire caused by high Fe3+ content and the beryllium-treated yellow sapphire by naked eyes. Some advanced techniques to identify the Be heat-treated yellow sapphire have been practiced such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser-induced breakdown spectroscopy (LIBS). We measured the colors of naturals and beryllium-treated yellow sapphires by UV-Vis reflectance spectroscopy. The concentration of impurities in the sapphire samples was measured by the LA-ICP-MS technique. To compare the yellow colors of CIE L * a * b * and CIE L * C * h, we found that the lightness and hue values of the Be-treated yellow sapphires were higher than the natural yellow sapphires with high Fe3+ content.