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.

The blue color mechanism on sapphires from different gem deposits before and after heating under oxidizing atmosphere

“…Since the color observation by the naked eyes with GIA gem set could not be properly acceptable. Because of no red color appearance, we found that there were weak fluorescence due to low Cr 3+ in the samples [24]. The CIELAB color indices of sapphire samples using colorimeter as shown in table 2 presented L * , a * and b * .…”