Abstract
Heat treatment is an important method used to improve the value of sapphires. The identification of heat-treated sapphires is a significant and challenging subject in gemology. In this study, natural sapphire samples from Ratnapura, Sri Lanka, were heated at different temperatures from 900 °C to 1500 °C. Then, the samples were examined by FTIR and three-dimensional fluorescence spectrometry. When excited by 450 nm light, most natural samples emitted a fluorescence band between 540 nm and 560 nm. This fluorescence disappeared after low-temperature heat treatment. Therefore, the presence of fluorescence between 540 nm and 560 nm is evidence of unheated sapphires from Sri Lanka. Almost all of the samples emitted fluorescence centered at 470 nm after high-temperature treatment. Therefore, fluorescence at 470 nm indicates that the sapphires from Sri Lanka were treated at a high temperature. Three-dimensional fluorescence spectroscopy can serve as a method to identify heat-treated sapphires.
Highlights
Sapphires are a variety of the mineral corundum, mainly composed of Al2 O3
Most natural specimens cannot be used for this purpose due to the low quality of their color. To improve their color and greatly enhance their commercial value, sapphires are treated with different methods, among which heat treatment is the most important
The cause of 3309/3232 cm−1 peaks is believed to be related to OH [1], but none of the samples investigated in this work showed this absorption peak
Summary
Sapphires are a variety of the mineral corundum, mainly composed of Al2 O3. Their lustrous and colorful appearance, excellent durability and long mining history make them one of the most valuable gems in the market. Most natural specimens cannot be used for this purpose due to the low quality of their color To improve their color and greatly enhance their commercial value, sapphires are treated with different methods, among which heat treatment is the most important. Due to the huge difference in commercial value between natural and treated sapphires, it is important for gem laboratories to identify whether sapphires submitted by customers have been treated. We used traditional methods (photomicrograph and FTIR) and fluorescence spectroscopy to obtain characteristics of sapphires before and after heat treatment, finding new evidence of heated sapphires
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